Add Ptr and RawNew for migration (#751)

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/interpreter/dictionary.h

@@ -37,8 +37,8 @@ class Dictionary {
   // Associate the value v with key k in the dictionary.
   // Time complexity: O(1).
   auto Set(const K& k, const V& v) -> void {
-    head = global_arena->New<ListNode<std::pair<K, V>>>(std::make_pair(k, v),
-                                                        head);
+    head = global_arena->RawNew<ListNode<std::pair<K, V>>>(std::make_pair(k, v),
+                                                           head);
   }
 
   typedef ListNodeIterator<std::pair<K, V>> Iterator;