Add a function to C, C++, and Java Collator that returns a folded string, which
is defined such that
someCollator.compare(a, b) == 0 IF AND ONLY IF someCollator.fold(a) is binary
equal to someCollator.fold(b).
A folded string doesn't say anything about ordering, just identity. Useful for
matching, as in regular expressions.
In implementation, we will favor the simplest string for the folding, e.g.
lowercase, no accents, etc.