regex: implement equivalence classes.
We already had the meaty part in __collate_equiv_value() that came with locale re-work from DFBSD, but was unused. Modify it to our needs making it accept just the single wide character as argument.
With that in place, the change is pretty simple -- parse equivalence-class bracketed expression, put resulting primary collating value to new filed 'cequiv' in cset struct, don't let p_bracket() "optimize" the resulting singleton, and, finally, do the collating values comparison first one in CHIN().