Recently we made it so that wrapper types created with `hash_newtype`
were not general purpose hash types i.e., one could not easily hash
arbitrary data into them. We would like to do the same for tagged
wrapped hash types.
In `hashes` do:
- Create a new macro `sha256t_tag` that does just the tag/engine stuff
out of the `sha256t_hash_newtype` macro.
- Deprecate the `sha256t_hash_newtype` macro.
In `bitcoin` do:
- Use a combination of `sha256t_tag` and `hash_newtype` to create tagged
wrapped hash types.
Note that we do not add private helper functions `engine` and
`from_engine` to the tagged wrapper types as we do for legacy/segwit in
`sighash`. Can be done later if wanted/needed.
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