post walk order seems incorrect

[lread]

While looking at #69, I decided to add some tests to make sure any changes I made did not affect walk order.

To refresh my brain on pre-walk and post-walk traversal, I read:

• Tree traversals
• clojure's postwalk-demo
• clojure's prewalk-demo

Rewrite-clj's prewalk travesal seems to match the above descriptions, but postwalk traversal seems off.

(defn- walk-order-tester [walk-fn s]
 (let [zloc (base/of-string s)
       visits (atom [])] 
   (walk-fn zloc (fn [zloc]
                     (swap! visits conj (base/string zloc))
                     zloc))
      @visits))

;; prewalk looks good:
 (walk-order-tester w/prewalk "(1 (2 3 (4 5) 6 (7 8)) 9)")
  ;; => ["(1 (2 3 (4 5) 6 (7 8)) 9)"
  ;;     "1"
  ;;     "(2 3 (4 5) 6 (7 8))"
  ;;     "2"
  ;;     "3"
  ;;     "(4 5)"
  ;;     "4"
  ;;     "5"
  ;;     "6"
  ;;     "(7 8)"
  ;;     "7"
  ;;     "8"
  ;;     "9"]

;; but postwalk looks wrong
  (walk-order-tester w/postwalk "(1 (2 3 (4 5) 6 (7 8)) 9)")
  ;; => ["9"
  ;;     "8"
  ;;     "7"
  ;;     "(7 8)"
  ;;     "6"
  ;;     "5"
  ;;     "4"
  ;;     "(4 5)"
  ;;     "3"
  ;;     "2"
  ;;     "(2 3 (4 5) 6 (7 8))"
  ;;     "1"
  ;;     "(1 (2 3 (4 5) 6 (7 8)) 9)"]

For postwalk, I would have expected:

["1"
 "2"
 "3"
 "4"
 "5"
 "(4 5)"
 "6"
 "7"
 "8"
 "(7 8)"
 "(2 3 (4 5) 6 (7 8))"
 "9"
 "(1 (2 3 (4 5) 6 (7 8)) 9)"]