Use multi-bit field for regex character set

Use multi-bit field for regex character set

The /d, /l, and /u regex modifiers are mutually exclusive.  This patch
changes the field that stores the character set to use more than one bit
with an enum determining which one.  This data structure more
closely follows the semantics of their being mutually exclusive, and
conserves bits as well, and is better expandable.

A small API is added to set and query the bit field.

This patch is not .xs source backwards compatible.  A handful of cpan
programs are affected.