ADDED: in-buffer completions for arithmetic functions

ADDED: in-buffer completions for arithmetic functions

* sweep.pl (sweep_function_functors_collection/2)
(sweep_functions_collection/2): New predicates.
(sweep_context_callable_arg/4): Recognize arithmetic functions.

* sweeprolog.el (sweeprolog-arith-functor-completion-candidates)
(sweeprolog-arith-completion-candidates): New functions, used in...
(sweeprolog--atom-or-functor-completion-at-point): ...here.

* sweep.texi (Code Completion): Document arithmetic function completion.
* sweeprolog-tests.el: Test it.