Add pretty-print extensions for custom types

[sritchie]

Our types aren't caught by clojure.pprint now. We can fix this with an implementation like this example from Gen.clj:

(require '[clojure.pprint :as pprint])
(defmethod pprint/simple-dispatch Choice [^Choice c]
  #?(:clj (.write ^java.io.Writer *out* "#gen/choice ")
     :cljs (-write *out* "#gen/choice "))
  (pprint/simple-dispatch (.-v c)))