Cleanup headers

[yoshuawuyts]

Precursor to #335, follow-up to #315. This removes all the manual name, value, and apply methods defined on the typed headers in favor of the Header trait. This also adds conversions to easily go from &impl Header -> HeaderName / HeaderValue, which allows for more convenient use. Which also enabled us to remove a fair amount of code from the manual IntoHeaderValues implementations on various typed headers.

Example

before

let authz = BasicAuth::new(username, password);

let mut res = Response::new(200);
authz.apply(&mut res);                                         // variant 1, inverted logic
res.insert_header(authz.header_name(), authz.header_value());  // variant 2, clearer but verbose

after

let authz = BasicAuth::new(username, password);

let mut res = Response::new(200);
res.insert_header(&authz, &authz); // enabled by this PR

Future directions

This doesn't preclude changing Header::insert_header to take one Header argument rather than two separate ones. But at least under the current API it significantly simplifies usage patterns and removes lots of duplicates.

[yoshuawuyts]

Don't think much here should be controversial -- this is mostly just a big cleanup. Going to go ahead and merge.