Speed up -flatten-n a bit

* dash.el (-flatten-n): Don't maintain a list of intermediate
results.  Eta-reduce.
* dev/examples.el (-flatten-n): Check for destructive side effects.

Re: #373.

dash.el

@@ -727,7 +727,9 @@ See also: `-flatten-n'"
 
 See also: `-flatten'"
   (declare (pure t) (side-effect-free t))
-  (-last-item (--iterate (--mapcat (-list it) it) list (1+ num))))
+  (dotimes (_ num)
+    (setq list (apply #'append (mapcar #'-list list))))
+  list)
 
 (defun -concat (&rest lists)
   "Return a new list with the concatenation of the elements in the supplied LISTS."

dev/examples.el

@@ -344,7 +344,10 @@ new list."
     (-flatten-n 0 '(3 4)) => '(3 4)
     (-flatten-n 0 '((1 2) (3 4))) => '((1 2) (3 4))
     (-flatten-n 0 '(((1 2) (3 4)))) => '(((1 2) (3 4)))
-    (-flatten-n 1 '(((1 . 2)) ((3 . 4)))) => '((1 . 2) (3 . 4)))
+    (-flatten-n 1 '(((1 . 2)) ((3 . 4)))) => '((1 . 2) (3 . 4))
+    (let ((l (list 1 (list 2) 3))) (-flatten-n 0 l) l) => '(1 (2) 3)
+    (let ((l (list 1 (list 2) 3))) (-flatten-n 1 l) l) => '(1 (2) 3)
+    (let ((l (list 1 (list 2) 3))) (-flatten-n 2 l) l) => '(1 (2) 3))
 
   (defexamples -replace
     (-replace 1 "1" '(1 2 3 4 3 2 1)) => '("1" 2 3 4 3 2 "1")