ENHANCED: faster completion-at-point for predicates

ENHANCED: faster completion-at-point for predicates

Filter predicate completion candidates according to the text near
point before formatting and computing argument names.  This makes
completion-at-point for predicates much faster when there are lots of
known predicates.

* sweeprolog.el (sweeprolog-predicate-completion-at-point): Pass
(partial) functor at point to...
* sweep.pl (sweep_predicate_completion_candidates/2): Filter
predicates based on new Sub argument.