- create ActorSystemImpl trait to make ActorSystem fully abstract
- add Java API for constructing (ActorSystem.create(...))
- only go through factory methods because .start() has become necessary
- rename all user-facing occurrences of “app” to “system” (Actor trait
and TestKit/AkkaSpec)
- pass ActorSystemImpl to ActorRefs upon creation, which means that
actorOf() and friends need such an argument, which must be provided to
the ActorRefProvider by the ActorRefFactory implementation
