Unicode: add ToFold mapping. Not used yet; but basically

Unicode: add ToFold mapping.  Not used yet; but basically
a more useful mapping for caseless aka case-ignoring than
doing either lc($a) eq lc($b) or uc($a) eq uc($b); the full
algorithm for creating the foldings uses equivalence classes,
see http://www.unicode.org/unicode/reports/tr21/
Hopefully this feature will be used in //i.
(The folding tables were introduced by #12689.)

p4raw-id: //depot/perl@12807