Revert --map to using mapcar

Code in the wild relies on mapcar for working with non-list
sequences, and mapcar can still be faster in some degenerate cases.

* dash.el (--map): Use mapcar again.

Fixes #365.

dash.el

@@ -207,14 +207,7 @@ Each element of LIST in turn is bound to `it' before evaluating
 BODY.
 This is the anaphoric counterpart to `-map'."
   (declare (debug (form form)))
-  (let ((l (make-symbol "list"))
-        (r (make-symbol "res")))
-    `(let ((,l ,list) ,r it)
-       (ignore it)
-       (while ,l
-         (setq it (pop ,l))
-         (push ,form ,r))
-       (nreverse ,r))))
+  `(mapcar (lambda (it) (ignore it) ,form) ,list))
 
 (defmacro --reduce-from (form init list)
   "Accumulate a value by evaluating FORM across LIST.