Implement common trait(s) to expose common traits of HTTP message (header) types

All HTTP messages in the http crate contain a bunch of shared data: the version, the headers and the extensions. Both the Request and the Response also allow callers to separate the headers part of a message from the body, resulting in two Parts types that both carry all three of these fields, so there are 4 types in total that share the same underlying data. It would be nice if there was a way to abstract over this data.

For my purposes, I mostly care about the HeaderMap<HeaderValue> -- my direct use case is that I have a function that makes it easier to extract the header's value as a &str and would like to apply this easily to both a request::Parts (which our framework uses internally to carry around request state) and a Response -- but it might make sense to take care of all three in one interface.

We could just have four impl AsRef<HeaderMap<HeaderValue>> for all four of these types as a minimal solution, or we could implement a custom trait, maybe like this:

trait HttpMessage {
     fn version(&self) -> Version;
     fn headers(&self) -> &HeaderMap<HeaderValue>;
     fn extensions(&self) -> &Extensions;
}

No source files, tests, or entry points are named. Start by comparing the shared version, headers, and extensions data on Request, Response, request::Parts, and response::Parts. Done means agreeing on and implementing a common interface, or the proposed AsRef implementations, with coverage for the stated request-parts and response use cases.