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.

Bắt đầu với HeaderName::from_static và đường dẫn contains_key được mô tả trong issue, sau đó so sánh hành vi hashing của StandardHeader và các header tùy chỉnh. Sử dụng benchmark TechEmpower được liên kết và các flamegraph để đo chi phí hiện tại; được xem là hoàn thành khi có một phương pháp hashing tại thời điểm biên dịch đã được review, kèm bằng chứng benchmark và các đánh đổi đã được thống nhất.