Use Bytes to represent custom headers

[caspervonb]

Use case

HeaderNames are well known within a program, if we use bytes to represent header names we can make use of from_static to allocate static header names.

Proposed change

Introduce from_static on HeaderName and make the internal representation for custom header names Bytes.

let header_name = HeaderName::from_static("X-Custom-Foo");

Internal representation would be

struct Custom {
  inner: Bytes
}

Who benefits from the change(s)?

Everyone

Alternative Approaches

No response