Refactor is_XDIGIT_uni(), is_XDIGIT_utf8() and macros

Refactor is_XDIGIT_uni(), is_XDIGIT_utf8() and macros

This adds macros to regen/regcharclass.pl that are usable as part of the
is_XDIGIT_foo() macros in handy.h, so that no function call need be done
to handle above Latin1 input.  These macros are quite small, and
unlikely to grow over time.  The functions that implement these in
utf8.c are also changed to use the macros instead of generating a swash.
This should speed things up slightly, with less memory used over time as
the swash fills.