Don't distort character ranges in rx translation

Don't distort character ranges in rx translation

The Emacs regexp engine interprets character ranges from ASCII to raw
bytes, such as [a-\xfe], as not including non-ASCII Unicode at all;
ranges from non-ACII Unicode to raw bytes, such as [ü-\x91], are
ignored entirely.

To make rx produce a translation that works as intended, split ranges
that that go from ordinary characters to raw bytes. Such ranges may
appear from set manipulation and regexp optimisation.

* lisp/emacs-lisp/rx.el (rx--generate-alt): Split intervals that
straddle the char-raw boundary when rendering a string regexp from an
interval set.
* test/lisp/emacs-lisp/rx-tests.el (rx-char-any-raw-byte):
Add test cases.