Create header name/value from String without reallocating

[glueball]

Proposed change

Implement IntoHeaderName and IntoHeaderValue for an (owned) String, so no allocation is necessary (*).

(*) For HeaderName, the underlying does not allocate if the Vec has not leftover capacity (https://docs.rs/bytes/latest/src/bytes/bytes.rs.html#849-882), but still this is the best we can do. However, I assume that for HeaderName using &'static str is the more common case.

Use case

I have to use format! to build some complex HeaderValue. So far, I have to use my_value.as_str().into_header_value() which internally uses .to_string(), so it creates a new allocation for the string, and the old allocation gets just freed afterward. Maybe the compiler can optimize this allocation away, but maybe not...

Contribution

Yes. Preparing PR.