Performance: re_matchsub/3

[kamahen]

See the discussion here: https://swi-prolog.discourse.group/t/careful-with-sub-atom-5-and-feature-request-last-sub-atom-5/5422/8 et seq.

Creating the results dict for this example is about equal to the time to do the matching. (From code inspection, re_match/2 and re_matchsub/3 differ only in creating the results dict.)