Feature request: compile-time hashes for `HeaderName::from_static`

Hi!
An almost complete rust newbie here (coming from C++, mostly), so please forgive my ignorance.

I believe it's a pretty common practice to have some company-wide standard headers for, say, tracing/authorization/etc., which are de-facto StandardHeaders in that sense.
However, since they aren't actually values of that enum, hashing them calls into FNV (i assume hashing a StandardHeader only hashes its discriminant, am i right?), and that FNV usage is measurable.

For example, i took the axum implementation from TechEmpower benchmarks, and did this: https://github.com/itrofimow/FrameworkBenchmarks/pull/7/files
Looking at flamegraphs, the contains_key amounts to ~3% of total CPU usage, most of it being spent in FNV, when replacing the CUSTOM_HEADERS with [ACCESS_CONTROL_ALLOW_CREDENTIALS, ACCESS_CONTROL_ALLOW_HEADERS, ACCESS_CONTROL_EXPOSE_HEADERS] (something of comparable length, basically) leads to the contains_key only taking ~0.3%.

Does it make sense to implement the optimization proposed, or do its drawbacks outweigh its performance benefits?
I have a clear understanding of how that could be done in C++ (we actually do so in the web-framework of ours, with its HeaderMap implementation being heavily inspired by what hyper offers), and would be glad to give it a try in Rust.

Comece com HeaderName::from_static e o caminho de contains_key descrito na issue; em seguida, compare o comportamento do hashing de StandardHeader e dos headers personalizados. Use o benchmark do TechEmpower vinculado e os flamegraphs para medir o custo atual; considera-se concluído quando houver uma abordagem de hashing em tempo de compilação revisada, com evidências de benchmark e trade-offs acordados.