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/allow.rs

@@ -1,7 +1,7 @@
 use header::{Header, HeaderFormat};
 use method::Method;
 use std::fmt::{self};
-use header::shared::util::{from_comma_delimited, fmt_comma_delimited};
+use header::parsing::{from_comma_delimited, fmt_comma_delimited};
 
 /// The `Allow` header.
 /// See also https://tools.ietf.org/html/rfc7231#section-7.4.1