regcomp.c: Use compiled-in inversion lists

This uses the compiled inversion lists to generate Posix character
classes and things like \v, \s inside bracketed character classes.

This paves the way for future optimizations, and fixes the bug which has
no formal bug number that /[[:ascii:]]/i matched non-Ascii characters,
such as the Kelvin sign, unlike /\p{ascii}/i.

pod/perldelta.pod

@@ -629,6 +629,13 @@ with "\n".  This has been fixed [perl #109206].
 
 =item *
 
+C<m/[[:ascii:]]/i> and C</\p{ASCII}/i> now match identically (when not
+under a differing locale).  This fixes a regression introduced in 5.14
+in which the first expression could match characters outside of ASCII,
+such as the KELVIN SIGN.
+
+=item *
+
 Method calls whose arguments were all surrounded with C<my()> or C<our()>
 (as in C<<$object->method(my($a,$b)) >>) used to force lvalue context on
 the subroutine.  This would prevent lvalue methods from returning certain