Without this patch, parse-commit will unconditionally use a faillure
continuation which simply returns `#f`. This may be undesirable in
some situations. As such, this commit allows (optionally) passing
a custom failure continuation as a second argument. If none is passed
the old behavior is used, hence this commit doesn't cause any backwards
incompatible API changes.
See #822
While testing a `(chibi parser)`-based parser I noticed that
`parse-string` is the only provided parser combinator which uses a list,
instead of a string, as a failure reason. It is not explicitly
documented but since all other standard parser combinator use a string
for the error reason I assume this to be a bug and have adjusted the
`parse-string` combinator accordingly in this commit.
