regexec.c: Revamp the macros to load swashes

regexec.c: Revamp the macros to load swashes

This changes the swash-load macros to use swash_init() and swash_fetch()
instead of calling is_utf8_xxx() functions that may only be needed for
this purpose (which will hence become eligible for removal because of
this commit).

The check that a known character matches the loaded swash is now only
done in DEBUGGING builds.  And the macro to load the DIGIT swash is
removed, as there are no remaining calls of it.