We manually implement these methods (and the GeneralHash trait) on newtypes
around sha256t::Hash, because tagged hashes require a bit more work. In
the next commit (API diff) you will see that this affects two hashes,
which are the only things that appear green in the diff.
Users who want to implement their own engine/from_engine types now need
to do it on their own. We do this for the non-Taproot sighash types in
`bitcoin` (though only privately) to demonstrate that it's possible.
91265977f8