Only invoke a given cached partial once

Note that this is backed by a LRU cache (which we soon shall see more usage of), so if you're a heavy user of cached partials it may be evicted and
refreshed if needed. But in most cases every partial is only invoked once.

This commit also adds a timeout (the global `timeout` config option) to make infinite recursion in partials
easier to reason about.

```
name              old time/op    new time/op    delta
IncludeCached-10    8.92ms ± 0%    8.48ms ± 1%   -4.87%  (p=0.016 n=4+5)

name              old alloc/op   new alloc/op   delta
IncludeCached-10    6.65MB ± 0%    5.17MB ± 0%  -22.32%  (p=0.002 n=6+6)

name              old allocs/op  new allocs/op  delta
IncludeCached-10      117k ± 0%       71k ± 0%  -39.44%  (p=0.002 n=6+6)
```

Closes #4086
Updates #9588

1 files changed, 45 insertions, 0 deletions

diff --git a/identity/identityhash_test.go b/identity/identityhash_test.go
new file mode 100644
index 000000000..378c0160d
--- /dev/null
+++ b/identity/identityhash_test.go
@@ -0,0 +1,45 @@
+// Copyright 2023 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 identity
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestHashString(t *testing.T) {
+	c := qt.New(t)
+
+	c.Assert(HashString("a", "b"), qt.Equals, "2712570657419664240")
+	c.Assert(HashString("ab"), qt.Equals, "590647783936702392")
+
+	var vals []any = []any{"a", "b", tstKeyer{"c"}}
+
+	c.Assert(HashString(vals...), qt.Equals, "12599484872364427450")
+	c.Assert(vals[2], qt.Equals, tstKeyer{"c"})
+
+}
+
+type tstKeyer struct {
+	key string
+}
+
+func (t tstKeyer) Key() string {
+	return t.key
+}
+
+func (t tstKeyer) String() string {
+	return "key: " + t.key
+}