refactor(headers): Introduce header!() macro, improve documentation

The new macro handles single value headers, list headers, and list
headers with at least one item.
It creates the item for the header and contains its documentation. The
new macro allows handling
more header cases in the future, it will also be possible to include
tests inside the macro.

BREAKING CHANGE: Removed impl_header!() and impl_list_header!() macros,
use new header!() macro.

src/header/common/transfer_encoding.rs

@@ -1,24 +1,20 @@
 use header::Encoding;
 
-/// The `Transfer-Encoding` header.
-///
-/// This header describes the encoding of the message body. It can be
-/// comma-separated, including multiple encodings.
-///
-/// ```notrust
-/// Transfer-Encoding: gzip, chunked
-/// ```
-///
-/// According to the spec, if a `Content-Length` header is not included,
-/// this header should include `chunked` as the last encoding.
-///
-/// The implementation uses a vector of `Encoding` values.
-#[derive(Clone, PartialEq, Debug)]
-pub struct TransferEncoding(pub Vec<Encoding>);
-
-impl_list_header!(TransferEncoding,
-                  "Transfer-Encoding",
-                  Vec<Encoding>);
+header! {
+    #[doc="`Transfer-Encoding` header, defined in"]
+    #[doc="[RFC7230](http://tools.ietf.org/html/rfc7230#section-3.3.1)"]
+    #[doc=""]
+    #[doc="The `Transfer-Encoding` header field lists the transfer coding names"]
+    #[doc="corresponding to the sequence of transfer codings that have been (or"]
+    #[doc="will be) applied to the payload body in order to form the message"]
+    #[doc="body."]
+    #[doc=""]
+    #[doc="# ABNF"]
+    #[doc="```plain"]
+    #[doc="Transfer-Encoding = 1#transfer-coding"]
+    #[doc="```"]
+    (TransferEncoding, "Transfer-Encoding") => (Encoding)+
+}
 
 bench_header!(normal, TransferEncoding, { vec![b"chunked, gzip".to_vec()] });
 bench_header!(ext, TransferEncoding, { vec![b"ext".to_vec()] });