RFC: A better data structure for `Headers`

Spinning out of discussion in https://github.com/http4s/http4s/pull/6201. cc @wjoel @sideeffffect @CremboC

Off the top of my head there are a few issues at play I believe:

1. A better data structure than List for appending new headers.
2. A better data structure than List for looking up headers.
3. A data structure that doesn’t preserve order, since it’s not required by spec and other frameworks don’t.

[bplommer]

So Map[CIString, NonEmptyChain[String]]?

[armanbilge]

@sideeffffect:

• https://github.com/typelevel/cats/pull/4264#issuecomment-1172973562

so my read is that Chain offers significant wins over List and Vector for concatenation heavy workloads (the accumulate benchmark) and that it is nearly as fast or faster than List for other cases.