Forbid non-ascii field names

[wingo]

Right now, the field-name production is a string which is specified to be compared case-insensitively with regards to equality. Case-insensitive comparisons are cheap to do for ASCII, but a bit annoying for full unicode. Fortunately (?), field names are always ASCII; values are not but that's OK. So, what about adding a restriction in wasi:http that forbids non-ascii field names?

[wingo]

Alternately, one could specify field names as list<u8>, and use the byte-lowercase algorithm from fetch: https://infra.spec.whatwg.org/#byte-lowercase

[lukewagner]

If we restrict field-name to RFC 9110's field-name as discussed in #178, then I believe we end up with a subset of 7-bit ASCII?