Fix some multi-char /i fold bugs

Fix some multi-char /i fold bugs

Consider U+FB05 and U+FB06.  These both fold to 'st', and hence should
match each other under /i.  However, Unicode doesn't furnish a rule for
this, and Perl hasn't been smart enought to figure it out.  The bug that
shows up is in constructs like

    "\x{fb06}" =~ /[^\x{fb05}]/i

succeeding.  Most of these instances also have a 'S' entry in Unicode's
CaseFolding.txt, which avoids the problem (as mktables was earlier
changed to include those in the generated table).  But there were
several code points that didn't.

This patch changes utf8.c to look for these when constructing it's
inverted list of case fold equivalents.  An alternative would have been
to change mktables instead to look for them and create synthetic rules.
But, this is more general in case the function ends up being used for
other things.

I will change fold_grind.t to test for these in a separate commit.