"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".
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