The http core types must define a correct header multimap.

Some good work has already been done by @daurnimator and other contributors in the standard library, and I feel like it would be great to embed it within the Zig HTTP core types library.

Pros:

• Do not rewrite something from scratch

Cons:

• Does not have yet a non-allocating API (reported in https://github.com/ziglang/zig/issues/5875)
• Some implementation details feels unecessary to me (cf: never-indexed fields), but I my understanding of the HTTP protocol is very limited to be really confident here :/