Define an engine type when creating tagged hash engines #2719
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In `bitcoin` we never use the `sha256t::Hash` type directly but only as an underlying generic type that holds the tagging logic. To get tagged hash types we currently use the `sha256t_hash_newtype` macro. Assuming other users implement tagged hashing as we do in `rust-bitcoin` then the `sha256t::Hash` type currently serves only as a type to implement `Tag::engine` on - but as long as there exists a way to get the pre-tagged engine then we do not actually need the type. If we assume all users will use a provided macro to define tagged hashes then we have two options. This PR implements the first option. 1. In the `NewTypeHash::engine` function return a pre-tagged `sha256::HashEngine`. 2. Have callers of the macro define a new engine type as well as a new hash type, then we can put the tagging logic in the `Default` implementation of the new engine. Remove the `sha256t::Hash` type, and the `Tag` trait. Modify the `sha256_hash_newtype` macro to match and just implement the tagging logic in the `engine` function. Add unit tests to make sure we correctly handle the forward/backward attribute. `rust-bitcoin` includes test already that verify this patch.
It may be useful for new tagged hash types created with the `sha256t_hash_newtype` macro to also define a hash engine. This PR demonstrates doing so.
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Done on top of #2717, this is just a demo.
It may be useful for new tagged hash types created with the
sha256t_hash_newtypemacro to also define a hash engine.