Add `cl-constantly' function

* lisp/emacs-lisp/cl-lib.el (cl-constantly): Add Common Lisp
function missing (bug#21584).

lisp/emacs-lisp/cl-lib.el

@@ -560,4 +560,9 @@ of record objects."
    (t
     (advice-remove 'type-of #'cl--old-struct-type-of))))
 
+(defun cl-constantly (value)
+  "Return a function that takes any number of arguments, but returns VALUE."
+  (lambda (&rest _)
+    value))
+
 ;;; cl-lib.el ends here

test/lisp/emacs-lisp/cl-lib-tests.el

@@ -551,4 +551,9 @@
     (should cl-old-struct-compat-mode)
     (cl-old-struct-compat-mode (if saved 1 -1))))
 
+(ert-deftest cl-constantly ()
+  (should (equal (mapcar (cl-constantly 3) '(a b c d))
+                 '(3 3 3 3))))
+
+
 ;;; cl-lib-tests.el ends here