This is the first step towards more type-safety in Actor interactions,
comprising:
* generic ActorRef[T] that only accepts T messages
* generic ActorSystem[T] extends ActorRef[T] (sending to the guardian,
whose Props[T] are provided for ActorSystem construction)
* removed the Actor trait: everything in there has been made into
messages and signals
* new Behavior[T] abstraction that consumes messages (of type T) or
Signals (lifecycle hooks, Terminated, ReceiveTimeout, Failed),
producing the next Behavior[T] as the result each time
* the ask pattern is provided and yields properly typed Futures
* variants of ActorContext are provided for synchronous testing of
Behaviors
All of this is implemented without touching code outside akka-typed
(apart from making guardianProps configurable), creating wrapper objects
around ActorRef, ActorContext, ActorSystem, Props and providing an Actor
implementation that just runs a Behavior.
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