(async) Remove or replace typed headers

Typed headers were removed between hyper 0.10 and hyper 0.12. typed-headers does not appear to have all the headers hyper 0.10 did, but it looks like hyperx and headers might.

Options:

• Re-export headers from one or the other library - this is what we did with hyper 0.10
• Remove typed header support - this is currently implemented in the async branch; only the header names are available in hyper and we re-export those.
• Implement From<X> for Header for every header in one or both of hyperx and typed-headers, perhaps behind a feature flag