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/origin.rs

@@ -1,4 +1,4 @@
-use header::{Header, Host};
+use header::{Header, Raw, Host};
 use std::fmt;
 use std::str::FromStr;
 use header::parsing::from_one_raw_str;
@@ -57,7 +57,7 @@ impl Header for Origin {
         NAME
     }
 
-    fn parse_header(raw: &[Vec<u8>]) -> ::Result<Origin> {
+    fn parse_header(raw: &Raw) -> ::Result<Origin> {
         from_one_raw_str(raw)
     }
 
@@ -100,10 +100,10 @@ mod tests {
 
     #[test]
     fn test_origin() {
-        let origin = Header::parse_header([b"http://foo.com".to_vec()].as_ref());
+        let origin = Header::parse_header(&vec![b"http://foo.com".to_vec()].into());
         assert_eq!(origin.ok(), Some(Origin::new("http", "foo.com", None)));
 
-        let origin = Header::parse_header([b"https://foo.com:443".to_vec()].as_ref());
+        let origin = Header::parse_header(&vec![b"https://foo.com:443".to_vec()].into());
         assert_eq!(origin.ok(), Some(Origin::new("https", "foo.com", Some(443))));
     }
 }