rx: Improve 'or' compositionality (bug#37659)

Perform 'regexp-opt' on nested 'or' forms, and after expansion of
user-defined and 'eval' forms.  Characters are now turned into strings
for wider 'regexp-opt' scope.  This preserves the longest-match
semantics for string in 'or' forms over composition.

* doc/lispref/searching.texi (Rx Constructs): Document.
* lisp/emacs-lisp/rx.el (rx--normalise-or-arg)
(rx--all-string-or-args): New.
(rx--translate-or): Normalise arguments first, and check for strings
in subforms.
(rx--expand-eval): Extracted from rx--translate-eval.
(rx--translate-eval): Call rx--expand-eval.
* test/lisp/emacs-lisp/rx-tests.el (rx-or, rx-def-in-or): Add tests.
* etc/NEWS: Announce.
1 job for emacs-27 in 54 minutes and 9 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #5121


Name Stage Failure
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 2
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check-expensive] Error 2
Makefile:294: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1