Consider returning [Substring] from HPACKHeaders.subscript(canonicalForm:)

Both `HTTPHeaders` and `HPACKHeaders` have very similar APIs. One slight difference is that `HTTPHeaders` `subscript(canonicalForm:)` returns `[Substring]` while `HPACKHeaders` returns `[String]`.

Returning `[Substring]` would allow us to potentially avoid some heap allocations as well as bringing the API more in line with `HTTPHeaders`.

We should consider adding an alternative subscript API now or at least changing this in the next major version of NIO HTTP/2.

Locate HPACKHeaders.subscript(canonicalForm:) and compare its API with HTTPHeaders.subscript(canonicalForm:), as described in the issue. Check how the return type is exposed and used, then determine whether an alternative subscript or a next-major-version change is appropriate; done means the HPACKHeaders API can return [Substring] without breaking the intended compatibility plan.

Written by the indexing model from the issue text.