Move ANYOF folding from regexec to regcomp

Move ANYOF folding from regexec to regcomp

This is for security as well as performance.  It allows Unicode properties to
not be matched case sensitively.  As a result the swash inversion hash is
converted from having utf8 keys to numeric, code point, keys.

It also for the first time fixes the bug where /i doesn't work for a code point
not at the end of a range in a bracketed character class has a multi-character
fold