Use compiled-in C structure for inverted case folds

Use compiled-in C structure for inverted case folds

This commit changes to use the C data structures generated by the
previous commit to compute what characters fold to a given one.  This is
used to find out what things should match under /i.

This now avoids the expensive start up cost of switching to perl
utf8_heavy.pl, loading a file from disk, and constructing a hash from
it.