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.
