regcomp.c: Optimize e.g., /[^\w]/, /[[^:word:]]/ into /\W/

regcomp.c: Optimize e.g., /[^\w]/, /[[^:word:]]/ into /\W/

This optimizes character classes that have a single element that is one
of the ops that have the same meaning outside (namely \d, \h, \s, \w,
\v, :word:, :digit: and their complements) to that op.  Those
ops take less space than a character class and run faster.   An initial
'^' for complementing the class is also handled.