regcomp.sym: Add ANYOFV node

regcomp.sym: Add ANYOFV node

This node is like a straight ANYOF node to match [bracketed character classes],
but can match multiple characters; in particular it can match a multi-char
fold.

When multi-char Unicode folding was added to Perl, it was overlooked that the
ANYOF node is supposed to match exactly one character, hence there have been
bugs ever since.  Adding a specialized node that can match multiple chars,
these can be fixed more easily.  I tried at first to make ANYOF match multiple
chars, but this causes Perl to not be able to fully compile.