feat(header): introduce header::Raw (#869)

The Raw type repesents the raw bytes of a header-value.

Having a special type allows a couple of benefits:

- The exact representation has become private, allowing "uglier"
internals. Specifically, since the common case is for a header to only
have 1 line of bytes, an enum is used to skip allocating a Vec for only
1 line. Additionally, a Cow<'static, [u8]> is used, so static bytes
don't require a copy. Finally, since we can use static bytes, when
parsing, we can compare the incoming bytes against a couple of the most
common header-values, and possibly remove another copy.

- As its own type, the `Headers.set_raw` method can be generic over
`Into<Raw>`, which allows for more ergnomic method calls.

BREAKING CHANGE: `Header::parse_header` now receives `&Raw`, instead of
  a `&[Vec<u8>]`. `Raw` provides several methods to ease using it, but
  may require some changes to existing code.

src/header/common/if_none_match.rs

@@ -67,10 +67,10 @@ mod tests {
     fn test_if_none_match() {
         let mut if_none_match: ::Result<IfNoneMatch>;
 
-        if_none_match = Header::parse_header([b"*".to_vec()].as_ref());
+        if_none_match = Header::parse_header(&b"*".as_ref().into());
         assert_eq!(if_none_match.ok(), Some(IfNoneMatch::Any));
 
-        if_none_match = Header::parse_header([b"\"foobar\", W/\"weak-etag\"".to_vec()].as_ref());
+        if_none_match = Header::parse_header(&b"\"foobar\", W/\"weak-etag\"".as_ref().into());
         let mut entities: Vec<EntityTag> = Vec::new();
         let foobar_etag = EntityTag::new(false, "foobar".to_owned());
         let weak_etag = EntityTag::new(true, "weak-etag".to_owned());