Add transient keyword to cached toString() value in _str

_str field in Keyword and Symbol classes lazily caches result of toString(). Because this field is not transient, serializing (using Java serialization) any keyword or symbol before and after calling toString() for the first time yields different results:

(import (java.io ByteArrayOutputStream ObjectOutputStream
                 ByteArrayInputStream  ObjectInputStream))

(defn- serialize [obj]
  (with-open [bos (ByteArrayOutputStream.)
              stream (ObjectOutputStream. bos)]
    (.writeObject stream obj)
    (-> bos .toByteArray seq)))

;; keyword example

(def k1 (serialize :k))
(println :k)
(def k2 (serialize :k))

(= k1 k2) ;;=> false 

;; symbol example

(def sym 'a)

(def s1 (serialize sym))
(println sym)
(def s2 (serialize sym))

(= s1 s2) ;;=> false

This issue came up when I was trying to use keywords as key in [Hazelcast](https://github.com/hazelcast/hazelcast) map. Hazelcast uses serialized keys in various scenarios, thus if I first put something to map under key :k and then print :k, I can no longer find such key.

Approach: Add transient keyword to _str field in Keyword and Symbol classes

Patch: CLJ-1408-3.patch

Screened by: Brenton Ashworth