Convert REGNODE_{SIMPLE,VARIES} to a bitmask lookup, from a strchr() lookup.

Convert REGNODE_{SIMPLE,VARIES} to a bitmask lookup, from a strchr() lookup.

This is O(1) with no branching, instead of O(n) with branching.
Deprecate the old implementation's externally visible variables
PL_simple and PL_varies. Google codesearch suggests that nothing outside the
core regexp code was using these.