refactor(hashKey): don't generate memory garbage

we now store both the object type and the id as the hashkey and return it for all objects.

for primitives we still have to do string concatination because we can't use expandos on them to
store the hashkey

src/apis.js

@@ -14,21 +14,23 @@
  *         The resulting string key is in 'type:hashKey' format.
  */
 function hashKey(obj, nextUidFn) {
-  var objType = typeof obj,
-      key;
+  var key = obj && obj.$$hashKey;
 
-  if (objType == 'function' || (objType == 'object' && obj !== null)) {
-    if (typeof (key = obj.$$hashKey) == 'function') {
-      // must invoke on object to keep the right this
+  if (key) {
+    if (typeof key === 'function') {
       key = obj.$$hashKey();
-    } else if (key === undefined) {
-      key = obj.$$hashKey = (nextUidFn || nextUid)();
     }
+    return key;
+  }
+
+  var objType = typeof obj;
+  if (objType == 'function' || (objType == 'object' && obj !== null)) {
+    key = obj.$$hashKey = objType + ':' + (nextUidFn || nextUid)();
   } else {
-    key = obj;
+    key = objType + ':' + obj;
   }
 
-  return objType + ':' + key;
+  return key;
 }
 
 /**