WIP HeaderValue: make Cow<'static, str>

[Fishrock123]

This allows HeaderValue to borrow 'static strings, rather than make allocations.

Unfortunately, we can't specialize TryFrom or From, so this also adds a new crate-only from_static_str() -> HeaderValue.

[joshtriplett]

Most header names (and many header values) will be short; we might want to use something like kstring here, with a small-string optimization. kstring also supports Cow.

[jbr]

Most header names (and many header values) will be short; we might want to use something like kstring here, with a small-string optimization. kstring also supports Cow.

trillium's header name implementation does something like this, using smartcow which is just a cow for smartstring. However it might also make sense to use some sort of string interning library because there are only a handful of header names used in reality and they're repeated across requests