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.
