add ExtendedActorSystem to shield ActorSystemImpl

- add provider, guardian, systemGuardian and deathWatch to it
- make ActorSystemImpl extend ExtendedActorSystem
- use ExtendedActorSystem for creating extensions, thereby limiting the
  access extensions get to just those four published methods.

akka-actor-tests/src/test/java/akka/actor/JavaExtension.java

@@ -23,15 +23,15 @@ public class JavaExtension {
       return TestExtensionId.TestExtensionProvider;
     }
 
-    public TestExtension createExtension(ActorSystemImpl i) {
+    public TestExtension createExtension(ExtendedActorSystem i) {
       return new TestExtension(i);
     }
   }
 
   static class TestExtension implements Extension {
-    public final ActorSystemImpl system;
+    public final ExtendedActorSystem system;
 
-    public TestExtension(ActorSystemImpl i) {
+    public TestExtension(ExtendedActorSystem i) {
       system = i;
     }
   }