Replace most 'extern crate' directives with 'use'.

  - See #480.

89 files changed, 270 insertions, 327 deletions

diff --git a/buffered-reader/src/lib.rs b/buffered-reader/src/lib.rs
index 32765d74..1b08e73c 100644
--- a/buffered-reader/src/lib.rs
+++ b/buffered-reader/src/lib.rs
@@ -223,12 +223,6 @@
 //! [`BufReader`]: https://doc.rust-lang.org/stable/std/io/struct.BufReader.html
 //! [HTTP's chunk transfer encoding]: https://en.wikipedia.org/wiki/Chunked_transfer_encoding
 
-#[cfg(feature = "compression-deflate")]
-extern crate flate2;
-#[cfg(feature = "compression-bzip2")]
-extern crate bzip2;
-extern crate libc;
-
 use std::io;
 use std::io::{Error, ErrorKind};
 use std::cmp;
diff --git a/core/src/lib.rs b/core/src/lib.rs
index 5db7e6bf..b1b39d2e 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -18,8 +18,8 @@
 /// # }
 /// ```
 
-extern crate dirs;
-extern crate tempfile;
+use dirs;
+use tempfile;
 
 use std::fmt;
 use std::io;
diff --git a/ffi-macros/src/lib.rs b/ffi-macros/src/lib.rs
index db37daef..6a3d2386 100644
--- a/ffi-macros/src/lib.rs
+++ b/ffi-macros/src/lib.rs
@@ -5,15 +5,13 @@
 use std::collections::HashMap;
 use std::io::Write;
 
-extern crate lazy_static;
 use lazy_static::lazy_static;
-extern crate syn;
+use syn;
 use syn::parse_quote;
 use syn::spanned::Spanned;
-extern crate quote;
-extern crate proc_macro;
-extern crate proc_macro2;
-extern crate sha2;
+use proc_macro;
+use proc_macro2;
+use sha2;
 
 use proc_macro::TokenStream;
 use proc_macro2::TokenStream as TokenStream2;
diff --git a/ffi-macros/src/rust2c.rs b/ffi-macros/src/rust2c.rs
index 2e469703..d0e252f5 100644
--- a/ffi-macros/src/rust2c.rs
+++ b/ffi-macros/src/rust2c.rs
@@ -1,6 +1,6 @@
-extern crate syn;
+use syn;
 use syn::export::ToTokens;
-extern crate proc_macro2;
+use proc_macro2;
 
 const PREFIX: &'static str = "pgp_";
 
diff --git a/ffi/src/error.rs b/ffi/src/error.rs
index 26940111..935d4515 100644
--- a/ffi/src/error.rs
+++ b/ffi/src/error.rs
@@ -2,7 +2,7 @@
 
 use std::io;
 
-extern crate sequoia_openpgp as openpgp;
+use sequoia_openpgp as openpgp;
 use sequoia_core as core;
 pub use crate::openpgp::error::Status;
 
diff --git a/ffi/src/lib.rs b/ffi/src/lib.rs
index ab2819ae..45844ded 100644
--- a/ffi/src/lib.rs
+++ b/ffi/src/lib.rs</