Reconsider implementation of Equatable and Hashable for HPACKHeaders

For historical reasons, HPACKHeaders does not pay attention to order or indexability when considering equality and hashability. We should consider whether this is appropriate for this type, or whether we should make the definition more strict.

Start by locating HPACKHeaders and reviewing its Equatable and Hashable implementations. Examine how equality currently treats header order and indexability, then review the existing issue discussion before proposing a stricter definition. Done requires an agreed semantic definition and corresponding implementation and test changes.

Written by the indexing model from the issue text.