0.7.0

• actors can now delay message processing by going into a sleep state
  • can be achieved through actor.sleep(duration) on the ActorWrapper or by returning ActorState::Sleep(duration) from within the actor
• fixed a bug where an actor with a limited mailbox can get stuck on shutdown
• added send_timeout()
• added return Result<(), ActorSendError> to message sending functions
• actually enforce configured actor limits per thread_pool
• added get_available_actor_count_for_pool that allows user to retrieve how many actors can still be spawned on a given thread_pool
• added ShardedRouter that will consistently route messages to the targets
  • added get_id to Message trait
  • shards are reset whenever an actor is added/removed to the router
  • shard count is equal to the amount of targets times 5
• ActorFactory::new_actor now returns a result
  • result of type error will result in same behavior as a panic
• send_after now returns Result like send and send_timeout
• added stop_with_code to ActorSystem that stops the system with a user defined error code
• changed ActorResult to Result<ActorResult, Box<dyn Error>> and added on_error() to Actor trait