utf8_heavy: Guard against infinite recursion

utf8_heavy: Guard against infinite recursion

If things aren't just so, it could be that utf8_heavy calls something
which requires a pattern, such as split or just a pattern match that
ends up calling utf8_heavy again, ad infinitum.  When this happens,
memory gets eaten up and the machine grinds to a halt, likely requiring a
manual forced reboot.

To prevent this undesirable situation, utf8_heavy now stacks all its
calls in progress, and if any is a repeat, panics.