When splitting a string, sometimes the separator needs to be included.
Split will now optionally include the separator at the end of the left
side of the splits, i.e. it appears at the end of the splits, except
for the last one.
Specifically, for musig() descriptors, Split is used to separate a
musig() from any derivation path that follows it by splitting on the
closing parentheses. Since that parentheses is needed for Func() and
Expr(), Split() needs to preserve the end parentheses instead of
discarding it.
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