object_pool: Pull available_objs from end of queue #91

We don't really care what order we get objects, and this results in developing
nodes an order of magnitude faster.

shift() means the entire array needs to be rebuilt, whereas pop() just means
popping one from the linked list.

OZprivate/rawJS/OZTreeModule/src/util/object_pool.js

@@ -21,8 +21,7 @@ class ObjectPool {
         this.create_new_obj(i, i-prev_size);
       }
     }
-    let first_available = this.available_obj.shift();
-    return this.arr[first_available];
+    return this.arr[this.available_obj.pop()];
   }
   create_new_obj(index_in_arr, available_index) {
     this.arr[index_in_arr] = new this.ClassType();