feat!: return SendError from send methods allowing replies to be received blocking

[tqwewe]

ActorRef's send methods which receive replies now return SendResult, allowing message replies to be either received asyncronously, or blocking.

actor_ref.send(msg).await?; // Async recv
actor_ref.send(msg).blocking_recv()?; // Blocking recv

Closes #25

[vadixidav]

Probably good to be merged, but I can give this a try in a few hours with my code to see how it will work out.