In this commit, we extend the txscript package to support re-deriving
the PkScript of an output by looking at the input's signature
script/witness attempting to spend it. As of this commit, the only
supported types are P2SH, v0 P2WSH, and v0 P2WPKH.
This will serve useful to detect when a particular script has been spent
on-chain.
A set of test vectors has also been added for the supported script types
to ensure its correctness.
