Add three new regexp functions: regexp_matches, regexp_split_to_array,

and regexp_split_to_table. These functions provide access to the
capture groups resulting from a POSIX regular expression match,
and provide the ability to split a string on a POSIX regular
expression, respectively. Patch from Jeremy Drake; code review by
Neil Conway, additional comments and suggestions from Tom and
Peter E.

This patch bumps the catversion, adds some regression tests,
and updates the docs.