Introduce a tree map for all content

This commit introduces a new data structure to store pages and their resources.

This data structure is backed by radix trees.

This simplies tree operations, makes all pages a bundle,  and paves the way for #6310.

It also solves a set of annoying issues (see list below).

Not a motivation behind this, but this commit also makes Hugo in general a little bit faster and more memory effective (see benchmarks). Especially for partial rebuilds on content edits, but also when taxonomies is in use.

```
name                                   old time/op    new time/op    delta
SiteNew/Bundle_with_image/Edit-16        1.32ms ± 8%    1.00ms ± 9%  -24.42%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file/Edit-16    1.28ms ± 0%    0.94ms ± 0%  -26.26%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories/Edit-16      33.9ms ± 2%    21.8ms ± 1%  -35.67%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs/Edit-16            40.6ms ± 1%    37.7ms ± 3%   -7.20%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree/Edit-16        56.7ms ± 0%    51.7ms ± 1%   -8.82%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates/Edit-16      19.9ms ± 2%    18.3ms ± 3%   -7.64%  (p=0.029 n=4+4)
SiteNew/Page_collections/Edit-16         37.9ms ± 4%    34.0ms ± 2%  -10.28%  (p=0.029 n=4+4)
SiteNew/Bundle_with_image-16             10.7ms ± 0%    10.6ms ± 0%   -1.15%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16         10.8ms ± 0%    10.7ms ± 0%   -1.05%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16           43.2ms ± 1%    39.6ms ± 1%   -8.35%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16                 47.6ms ± 1%    47.3ms ± 0%     ~     (p=0.057 n=4+4)
SiteNew/Deep_content_tree-16             73.0ms ± 1%    74.2ms ± 1%     ~     (p=0.114 n=4+4)
SiteNew/Many_HTML_templates-16           37.9ms ± 0%    38.1ms ± 1%     ~     (p=0.114 n=4+4)
SiteNew/Page_collections-16              53.6ms ± 1%    54.7ms ± 1%   +2.09%  (p=0.029 n=4+4)

name                                   old alloc/op   new alloc/op   delta
SiteNew/Bundle_with_image/Edit-16         486kB ± 0%     430kB ± 0%  -11.47%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file/Edit-16     265kB ± 0%     209kB ± 0%  -21.06%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories/Edit-16      13.6MB ± 0%     8.8MB ± 0%  -34.93%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs/Edit-16            66.5MB ± 0%    63.9MB ± 0%   -3.95%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree/Edit-16        28.8MB ± 0%    25.8MB ± 0%  -10.55%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates/Edit-16      6.16MB ± 0%    5.56MB ± 0%   -9.86%  (p=0.029 n=4+4)
SiteNew/Page_collections/Edit-16         16.9MB ± 0%    16.0MB ± 0%   -5.19%  (p=0.029 n=4+4)
SiteNew/Bundle_with_image-16             2.28MB ± 0%    2.29MB ± 0%   +0.35%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16         2.07MB ± 0%    2.07MB ± 0%     ~     (p=0.114 n=4+4)
SiteNew/Tags_and_categories-16           14.3MB ± 0%    13.2MB ± 0%   -7.30%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16                 69.1MB ± 0%    69.0MB ± 0%     ~     (p=0.343 n=4+4)
SiteNew/Deep_content_tree-16             31.3MB ± 0%    31.8MB ± 0%   +1.49%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16           10.8MB ± 0%    10.9MB ± 0%   +1.11%  (p=0.029 n=4+4)
SiteNew/Page_collections-16              21.4MB ± 0%    21.6MB ± 0%   +1.15%  (p=0.029 n=4+4)

name                                   old allocs/op  new allocs/op  delta
SiteNew/Bundle_with_image/Edit-16         4.74k ± 0%     3.86k ± 0%  -18.57%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file/Edit-16     4.73k ± 0%     3.85k ± 0%  -18.58%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories/Edit-16        301k ± 0%      198k ± 0%  -34.14%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs/Edit-16              389k ± 0%      373k ± 0%   -4.07%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree/Edit-16          338k ± 0%      262k ± 0%  -22.63%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates/Edit-16        102k ± 0%       88k ± 0%  -13.81%  (p=0.029 n=4+4)
SiteNew/Page_collections/Edit-16           176k ± 0%      152k ± 0%  -13.32%  (p=0.029 n=4+4)
SiteNew/Bundle_with_image-16              26.8k ± 0%     26.8k ± 0%   +0.05%  (p=0.029 n=4+4)
SiteNew/Bundle_with_JSON_file-16          26.8k ± 0%     26.8k ± 0%   +0.05%  (p=0.029 n=4+4)
SiteNew/Tags_and_categories-16             273k ± 0%      245k ± 0%  -10.36%  (p=0.029 n=4+4)
SiteNew/Canonify_URLs-16                   396k ± 0%      398k ± 0%   +0.39%  (p=0.029 n=4+4)
SiteNew/Deep_content_tree-16               317k ± 0%      325k ± 0%   +2.53%  (p=0.029 n=4+4)
SiteNew/Many_HTML_templates-16             146k ± 0%      147k ± 0%   +0.98%  (p=0.029 n=4+4)
SiteNew/Page_collections-16                210k ± 0%      215k ± 0%   +2.44%  (p=0.029 n=4+4)
```

Fixes #6312
Fixes #6087
Fixes #6738
Fixes #6412
Fixes #6743
Fixes #6875
Fixes #6034
Fixes #6902
Fixes #6173
Fixes #6590

71 files changed, 4758 insertions, 2430 deletions

diff --git a/commands/convert.go b/commands/convert.go
index b9129e594..fe64405e9 100644
--- a/commands/convert.go
+++ b/commands/convert.go
@@ -16,10 +16,11 @@ package commands
 import (
 	"bytes"
 	"fmt"
-	"io"
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/parser/pageparser"
+
 	"github.com/gohugoio/hugo/resources/page"
 
 	"github.com/gohugoio/hugo/hugofs"
@@ -28,7 +29,6 @@ import (
 
 	"github.com/gohugoio/hugo/parser"
 	"github.com/gohugoio/hugo/parser/metadecoders"
-	"github.com/gohugoio/hugo/parser/pageparser"
 
 	"github.com/pkg/errors"
 
@@ -157,7 +157,7 @@ func (cc *convertCmd) convertAndSavePage(p page.Page, site *hugolib.Site, target
 		return nil
 	}
 
-	pf, err := parseContentFile(file)
+	pf, err := pageparser.ParseFrontMatterAndContent(file)
 	if err != nil {
 		site.Log.ERROR.Println(errMsg)
 		file.Close()
@@ -167,23 +167,23 @@ func (cc *convertCmd) convertAndSavePage(p page.Page, site *hugolib.Site, target
 	file.Close()
 
 	// better handling of dates in formats that don't have support for them
-	if pf.frontMatterFormat == metadecoders.JSON || pf.frontMatterFormat == metadecoders.YAML || pf.frontMatterFormat == metadecoders.TOML {
-		for k, v := range pf.frontMatter {
+	if pf.FrontMatterFormat == metadecoders.JSON || pf.FrontMatterFormat == metadecoders.YAML || pf.FrontMatterFormat == metadecoders.TOML {
+		for k, v := range pf.FrontMatter {
 			switch vv := v.(type) {
 			case time.Time:
-				pf.frontMatter[k] = vv.Format(time.RFC3339)
+				pf.FrontMatter[k] = vv.Format(time.RFC3339)
 			}
 		}
 	}
 
 	var newContent bytes.Buffer
-	err = parser.InterfaceToFrontMatter(pf.frontMatter, targetFormat, &newContent)
+	err = parser.InterfaceToFrontMatter(pf.FrontMatter, targetFormat, &newContent)
 	if err != nil {
 		site.Log.ERROR.Println(errMsg)
 		return err
 	}
 
-	newContent.Write(pf.content)
+	newContent.Write(pf.Content)
 
 	newFilename := p.File().Filename()
 
@@ -210,39 +210,3 @@ type parsedFile struct {
 	// Everything after Front Matter
 	content []byte
 }
-
-func parseContentFile(r io.Reader) (parsedFile, error) {
-	var pf parsedFile
-
-	psr, err := pageparser.Parse(r, pageparser.Config{})
-	if err != nil {
-		return pf, err
-	}
-
-	iter := psr.Iterator()
-
-	walkFn := func(item pageparser.Item) bool {
-		if pf.frontMatterSource != nil {
-			// The rest is content.
-			pf.content = psr.Input()[item.Pos:]
-			// Done
-			return false
-		} else if item.IsFrontMatter() {
-			pf.frontMatterFormat = metadecoders.FormatFromFrontMatterType(item.Type)
-			pf.frontMatterSource = item.Val
-		}
-		return true
-
-	}
-
-	iter.PeekWalk(walkFn)
-
-	metadata, err := metadecoders.Default.UnmarshalToMap(pf.frontMatterSource, pf.frontMatterFormat)
-	if err != nil {
-		return pf, err
-	}
-	pf.frontMatter = metadata
-
-	return pf, nil
-
-}
diff --git a/commands/import_jekyll.go b/commands/import_jekyll.go
index d2e7b27da..2dd0fc051 100644
--- a/commands/import_jekyll.go
+++ b/commands/import_jekyll.go
@@ -26,6 +26,8 @@ import (
 	"time"
 	"unicode"
 
+	"github.com/gohugoio/hugo/parser/pageparser"
+
 	"github.com/gohugoio/hugo/common/hugio"
 
 	"github.com/gohugoio/hugo/parser/metadecoders"
@@ -397,19 +399,19 @@ func convertJekyllPost(path, relPath, targetDir string, draft bool) error {
 		return err
 	}
 
-	pf, err := parseContentFile(bytes.NewReader(contentBytes))
+	pf, err := pageparser.ParseFrontMatterAndContent(bytes.NewReader(contentBytes))
 	if err != nil {
 		jww.ERROR.Println("Parse file error:", path)
 		return err
 	}
 
-	newmetadata, err := convertJekyllMetaData(pf.frontMatter, postName, postDate, draft)
+	newmetadata, err := convertJekyllMetaData(pf.FrontMatter, postName, postDate, draft)
 	if err != nil {
 		jww.ERROR.Println("Convert metadata error:", path)
 		return err
 	}
 
-	content, err := convertJekyllContent(newmetadata, string(pf.content))
+	content, err := convertJekyllContent(newmetadata, string(pf.Content))
 	if err != nil {
 		jww.ERROR.Println("Converting Jekyll error:", path)
 		return err
diff --git a/common/herrors/errors.go b/common/herrors/errors.go
index 5fae6fcae..fded30b1a 100644
--- a/common/herrors/errors.go
+++ b/common/herrors/errors.go
@@ -57,6 +57,11 @@ func PrintStackTrace(w io.Writer) {
 	fmt.Fprintf(w, "%s", buf)
 }
 
+// ErrorSender is a, typically, non-blocking error handler.
+type ErrorSender interface {
+	SendError(err error)
+}
+
 // Recover is a helper function that can be used to capture panics.
 // Put this at the top of a method/function that crashes in a template:
 //     defer herrors.Recover()
diff --git a/common/para/para_test.go b/common/para/para_test.go
index bda7f5d27..9b268b0c0 100644
--- a/common/para/para_test.go
+++ b/common/para/para_test.go
@@ -16,6 +16,7 @@ package para
 import (
 	"context"
 	"runtime"
+
 	"sort"
 	"sync"
 	"sync/atomic"
diff --git a/common/types/convert.go b/common/types/convert.go
new file mode 100644
index 000000000..b55330757
--- /dev/null
+++ b/common/types/convert.go
@@ -0,0 +1,28 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package types
+
+import "github.com/spf13/cast"
+
+// ToStringSlicePreserveString converts v to a string slice.
+// If v is a string, it will be wrapped in a string slice.
+func ToStringSlicePreserveString(v interface{}) []string {
+	if v == nil {
+		return nil
+	}
+	if sds, ok := v.(string); ok {
+		return []string{sds}
+	}
+	return cast.ToStringSlice(v)
+}
diff --git a/common/types/convert_test.go b/common/types/convert_test.go
new file mode 100644
index 000000000..7f86f4c8a
--- /dev/null
+++ b/common/types/convert_test.go
@@ -0,0 +1,29 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package types
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestToStringSlicePreserveString(t *testing.T) {
+	c := qt.New(t)
+
+	c.Assert(ToStringSlicePreserveString("Hugo"), qt.DeepEquals, []string{"Hugo"})
+	c.Assert(ToStringSlicePreserveString([]interface{}{"A", "B"}), qt.DeepEquals, []string{"A", "B"})
+	c.Assert(ToStringSlicePreserveString(nil), qt.IsNil)
+
+}
diff --git a/config/configProvider.go b/config/configProvider.go
index 187fb7b10..928bf948a 100644
--- a/config/configProvider.go
+++ b/config/configProvider.go
@@ -14,7 +14,7 @@
 package config
 
 import (
-	"github.com/spf13/cast"
+	"github.com/gohugoio/hugo/common/types"
 )
 
 // Provider provides the configuration settings for Hugo.
@@ -35,14 +35,7 @@ type Provider interface {
 // we do not attempt to split it into fields.
 func GetStringSlicePreserveString(cfg Provider, key string) []string {
 	sd := cfg.Get(key)
-	return toStringSlicePreserveString(sd)
-}
-
-func toStringSlicePreserveString(v interface{}) []string {
-	if sds, ok := v.(string); ok {
-		return []string{sds}
-	}
-	return cast.ToStringSlice(v)
+	return types.ToStringSlicePreserveString(sd)
 }
 
 // SetBaseTestDefaults provides some common config defaults used in tests.
diff --git a/create/content_template_handler.go b/create/content_template_handler.go
index e4cddedf5..3a7007f1a 100644
--- a/create/content_template_handler.go
+++ b/create/content_template_handler.go
@@ -110,7 +110,7 @@ func executeArcheTypeAsTemplate(s *hugolib.Site, name, kind, targetPath, archety
 		Date: time.Now().Format(time.RFC3339),
 		Name: name,
 		File: f,
-		Site: &s.Info,