gh-152033: Optimize category escapes outside character sets (GH-152035)

gh-152033: Optimize category escapes outside character sets (GH-152035)

Character class escapes (``\d``, ``\D``, ``\s``, ``\S``, ``\w`` and
``\W``) that occur outside a character set are now compiled directly to a
single CATEGORY opcode instead of being wrapped in an IN block.  This
removes the IN wrapper (three code words) and an indirect charset() call,
and makes such an escape a simple repeatable unit so that, for example,
``\d+`` uses the REPEAT_ONE fast path; a CATEGORY case is added to
SRE(count).

The transformation preserves behaviour exactly.  For category-heavy
patterns the compiled byte code is about 20% smaller and matching is up
to ~2x faster, with no effect on patterns that do not use bare category
escapes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>