These two functions are related. We cannot, by definition, get the
witness version from a script that is not a witness program but
currently the code is not linking these two things.
Refactor by doing:
- Move the check of the witness program bip rules to `witness_version`
- Call `witness_version().is_some()` in the predicate
Improve the docs while we are at it to include the bip text in the
rustdoc. Note I didn't bother referencing the segwit bip number, this
bip text is pretty well known.
dec05b63e9