"schnorr" is a dirty word; the current `schnorr` module defines a
`Signature` that includes a sighash type, this sighash type is a bitcoin
specific construct related to taproot. Therefore the `Signature` is
better named `taproot::Signature`. Note also that the usage of `schnorr`
in `secp256k1` is probably justified because the
`secp256::schnorr::Signature` is just doing the crypto.
While we are at it, update docs and error messages to use "taproot"
instead of "schnorr". Also change function names and identifiers that
use "schnorr".
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