bug#37659: rx additions: anychar, unmatchable, unordered-or

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Tue, 11 Feb 2020 13:57:27 +0100
> Cc: 37659@debbugs.gnu.org
> 
> Since the change was introduced in Emacs 27 which has not yet been released, 
> I suggest the attached patch for emacs-27. It reverts the use of regexp-opt 
> with KEEP-ORDER = t. What do you think? It would solve the problem without 
> introducing new constructs, and without running the risk of introducing 
> subtle errors in existing rx expressions.

Can't say I'm happy with these last-minute experiments, but okay.

Thanks.