pre-allocate the deferred's shape

This allows the VM to optimize correctly.

lib/rsvp/defer.js

 import { Promise } from "./promise";
 
 function defer() {
-  var deferred = {};
+  var deferred = {
+    // pre-allocate shape
+    resolve: undefined,
+    reject:  undefined,
+    promise: undefined
+  };
 
-  var promise = new Promise(function(resolve, reject) {
+  deferred.promise = new Promise(function(resolve, reject) {
     deferred.resolve = resolve;
     deferred.reject = reject;
   });
 
-  deferred.promise = promise;
   return deferred;
 }