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

[Paul Eggert]

On 2/13/20 10:38 AM, Mattias Engdegård wrote:
> Now the regexp-opt KEEP-ORDER argument no longer serves any purpose; it was 
> added for use by rx, and has no obvious use elsewhere. It could safely be 
> removed to save some weight, unless you prefer it be kept as scar tissue.

Simplest would be to remove it in Emacs 27, and merge that change into 
master.

If that's too drastic, we can mark it as deprecated in Emacs 27 (though 
it is odd for a release to add a feature that is immediately 
deprecated), and remove it in the master branch.