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

[glbrntt]

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.