alist-get: Add optional arg TESTFN

alist-get: Add optional arg TESTFN

If TESTFN is non-nil, then it is the predicate to lookup
the alist.  Otherwise, use 'eq' (Bug#27584).
* lisp/subr.el (alist-get): Add optional arg FULL.
* lisp/emacs-lisp/map.el (map-elt, map-put): Add optional arg TESTFN.
* lisp/emacs-lisp/gv.el (alist-get): Update expander.
* doc/lispref/lists.texi (Association Lists): Update manual.
* etc/NEWS: Announce the changes.
* test/lisp/emacs-lisp/map-tests.el (test-map-put-testfn-alist)
(test-map-elt-testfn): New tests.