Adjust arity of predicates according to completion context

Adjust arity of predicates according to completion context

* sweep.pl (sweep_context_callable): track required arity adjustment.
(sweep_predicate_completion_candidates): take arity difference as
argument, adjust and filter candidates according to it.
* sweeprolog.el (sweeprolog-predicate-completion-at-point): use it.
* sweeprolog-tests.el: test it.