In this commit, we add a new signatureVerifier interface that will allow
us to consolidate a lot of code as we'll now have 4 distinct sig+sighash
types to verify:
1. pre-segwit
2. segwit v0
3. segwit v1 (taproot key spend)
4. tapscript spends
We'll need to be able to handle 3 of the cases for the modified
OP_CHECKSIG operator. This new abstraction allows us to keep the
implementation of the function somewhat succinct.
In this commit we implement a verifier for #3 which is needed to verify
the top-level taproot keyspend. We expose the verifier using a new
VerifyTaprootKeySpend function.
