In this commit, we add more detail to the invalid tapscript merkle proof
error. Before this commit, the error was blank, making such a case hard
to debug. We'll now log the expected witness program, what we derived,
and also the passed in tapscript root.
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.
