Enqueue promise jobs from SpiderMonkey callbacks, and execute them in batches. Implement native Promise APIs.

Add SpiderMonkey hooks for enqueuing promise jobs. Start porting various native Promise APIs.

components/script/dom/webidls/Promise.webidl

@@ -5,7 +5,12 @@
 // This interface is entirely internal to Servo, and should not be accessible to
 // web pages.
 
-[NoInterfaceObject]
+callback PromiseJobCallback = void();
+
+[TreatNonCallableAsNull]
+callback AnyCallback = any (any value);
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
 // Need to escape "Promise" so it's treated as an identifier.
 interface _Promise {
 };