feat(header): introduce header::Raw

[seanmonstar]

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.

[seanmonstar]

cc @mitsuhiko