regexec.c: Don't give up on fold matching early

As noted in the comments of the code, "a" =~ /[A]/i doesn't work currently
(except that regcomp.c knows about the ASCII characters and corrects for
it, but not always, for example in cases like "a" =~ /\p{Upper}/i.  This
patch catches all those).

It works by computing a list of all characters that (singly) fold to
another one, and then checking each of those.  The maximum length of
the list is 3 in the current Unicode standard.

I believe that a better long-term solution is to do this at compile
rather than execution time, by generating a closure of everything
matched.  But this can't be done now because the data structure would
need to be extensively revamped to list all non-byte characters, and
user-defined \p{} matches are not known at compile-time.

And it doesn't handle the multi-char folds.  There is a separate ticket
for those.

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