In this commit, we fix an inadvertent mutation bug that would at times
cause the private key passed into the tweak function to actually be
*modified* in place.
We fix this by accepting the value instead of a pointer. The actual
private key struct itself contains no pointer fields, so this is
effectively a deep copy via dereference.
We also add a new unit test that fails w/o this change, to show that the
private key was indeed being modified.
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 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.
