Currently we have `TapSighash` that is used for taproot sighashes but
for non-taproot sighashes we use `hash_types::Sighash`. We can improve
the API by creating a `LegacySighash`, and `SegwitV0Sighash`.
Copy the original `Sighash` macro calls to create the two new types in
the `sighash` module.
While we are at it, put the `TapSighash` and `TapSighashTag` into the
`sighash` module also.
There is never any use for the `sighash` module unless one is signing,
which requires the `crypto` module. The `sighash` module should
therefore live in the `crypto` module. This is not an API breaking
change because we reexport it at the crate root.
Tobin C. Harding