rx performance improvements

rx performance improvements

* lisp/emacs-lisp/rx.el (rx--generate-alt):
Treat the intervals and classes lists separately without joining,
to reduce allocation.  Handle special cases first.
(rx--union-intervals):
Implement directly instead of using intersection and complement.
* test/lisp/emacs-lisp/rx-tests.el (rx-any): Adapt test, as some
character alternatives are now slightly different.
(rx--complement-intervals, rx--union-intervals)
(rx--intersect-intervals): New unit tests.