ADDED: New command 'sweeprolog-extract-region-to-predicate'

ADDED: New command 'sweeprolog-extract-region-to-predicate'

Add a command for extracting a part of a clause body into a separate
predicate.

* sweep.pl (sweep_term_variable_names/2)
(sweep_goal_may_cut/2): New helper predicates.
* sweeprolog.el (sweeprolog-extract-region-to-predicate): New command.
(sweeprolog-maybe-extract-region-to-predicate): New function.
(sweeprolog-insert-term-functions): Add it.
* sweep.texi (Insert Term DWIM, Extract Goal): Document it.