In this commit, we add a new AssembleTaprootScriptTree function that
given a list of tapscript leaves, generates a valid tapscript root,
along with the auxiliary proof data needed to spend each output.
In this commit, we add a new function `RawTxInTapscriptSignature` that
will be used to generate signatures in the _tapscript_ context. Note
that this differs from top-level taproot as a distinct sighash is used,
and we _always_ accept a root hash to perform the proper tweak.
In this commit, we add a new function to verify the taproot merkle
commitment of a given tapscript leaf. Along the way we add some helper
functions which can be used to construct a taproot output given the raw
script root.
In this commit, we add a new struct to represent the ControlBlock
structure used to feed in the tapscript leaf inclusion proof into the
witness tack. The `ParseControlBlock` parses a would-be control block
and returns an error if it's incorrectly formatted.
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.
