regen/regcharclass.pl: Simplify generated safe macros

regen/regcharclass.pl: Simplify generated safe macros

This simplifies the macros generated which make sure there are no read
errors.  Prior to this commit, the code generated looked like

      (e - s) > 3
      ? see if things of at most length 4 match
      : (e - s) > 2
        ? see if things of at most length 3 match
        : (e - s) > 1
          ? see if things of at most length 2 match
          : (e - s) > 0
            ? see if things of at most length 1 match

For things that are a single character, the ones greater than length 2
must be in UTF8, and their needed length can be determined by UTF8SKIP,
so we can get rid of most of the (e-s) tests.

This doesn't change the macros which can match multiple characters; that
is a harder to do.