refactor(headers): export all headers and utils directly under header

Currently headers are exported at many places. For example you can access
`Transfer-Encoding` header at `header`, `header::common` and
`header::common::transfer_encoding`. Per discussion on IRC with
@seanmonstar and @reem, all contents of headers will be exposed at `header`
directly. Parsing utilities will be exposed at `header::parsing`. Header
macros can now be used from other crates.

This breaks much code using headers. It should use everything it needs
directly from `header::`, encodings are exposed at `header::Encoding::`,
connection options are exposed at `header::ConnectionOption`.

src/header/common/transfer_encoding.rs

@@ -1,9 +1,7 @@
 use header::{Header, HeaderFormat};
 use std::fmt;
-use std::str::FromStr;
-use header::shared::util::{from_comma_delimited, fmt_comma_delimited};
-
-use self::Encoding::{Chunked, Gzip, Deflate, Compress, EncodingExt};
+use header::Encoding;
+use header::parsing::{from_comma_delimited, fmt_comma_delimited};
 
 /// The `Transfer-Encoding` header.
 ///
@@ -23,54 +21,6 @@ pub struct TransferEncoding(pub Vec<Encoding>);
 
 deref!(TransferEncoding => Vec<Encoding>);
 
-/// A value to be used with the `Transfer-Encoding` header.
-///
-/// Example:
-///
-/// ```
-/// # use hyper::header::TransferEncoding;
-/// # use hyper::header::transfer_encoding::Encoding::{Gzip, Chunked};
-/// # use hyper::header::Headers;
-/// # let mut headers = Headers::new();
-/// headers.set(TransferEncoding(vec![Gzip, Chunked]));
-#[derive(Clone, PartialEq, Show)]
-pub enum Encoding {
-    /// The `chunked` encoding.
-    Chunked,
-    /// The `gzip` encoding.
-    Gzip,
-    /// The `deflate` encoding.
-    Deflate,
-    /// The `compress` encoding.
-    Compress,
-    /// Some other encoding that is less common, can be any String.
-    EncodingExt(String)
-}
-
-impl fmt::String for Encoding {
-    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        write!(fmt, "{}", match *self {
-            Chunked => "chunked",
-            Gzip => "gzip",
-            Deflate => "deflate",
-            Compress => "compress",
-            EncodingExt(ref s) => s.as_slice()
-        })
-    }
-}
-
-impl FromStr for Encoding {
-    fn from_str(s: &str) -> Option<Encoding> {
-        match s {
-            "chunked" => Some(Chunked),
-            "deflate" => Some(Deflate),
-            "gzip" => Some(Gzip),
-            "compress" => Some(Compress),
-            _ => Some(EncodingExt(s.to_string()))
-        }
-    }
-}
-
 impl Header for TransferEncoding {
     fn header_name(_: Option<TransferEncoding>) -> &'static str {
         "Transfer-Encoding"
@@ -89,4 +39,3 @@ impl HeaderFormat for TransferEncoding {
 
 bench_header!(normal, TransferEncoding, { vec![b"chunked, gzip".to_vec()] });
 bench_header!(ext, TransferEncoding, { vec![b"ext".to_vec()] });
-