Fix merging of ambiguous nil maps

Fix merging of ambiguous nil maps

* lisp/emacs-lisp/map.el: Bump version to 3.1.
(map--merge): New merging subroutine that uses a hash table in place
of lists, for both efficiency and avoiding ambiguities (bug#49848).
(map-merge): Rewrite in terms of map--merge.
(map-merge-with): Ditto.  This ensures that FUNCTION is called
whenever two keys are merged, even if they are not eql (which could
happen until now).  It also makes map-merge-with consistent with
map-merge, thus achieving greater overall predictability.
* etc/NEWS: Announce this weakening of guarantees.
* test/lisp/emacs-lisp/map-tests.el (test-map-merge)
(test-map-merge-with): Don't depend on specific orderings.  Test
that nil is correctly merged into a plist.