feat: add utility methods to WeakActorRef

[tqwewe]

Adds the following new methods to WeakActorRef:

• is_current
• kill
• wait_for_startup variants
• wait_for_shutdown variants
• unlink variants
• mailbox_sender methods

Omits link creation and remote registration methods as these create lifecycle commitments inappropriate for weak reference semantics.

The wait_for_shutdown methods has a very slight possible behavioral difference with those from ActorRef, in that tokio's mpsc::WeakSender doesn't expose a .closed() method, like mpsc::Sender has. However, it still waits for the internal shutdown result to be set, which is effectively the same.

Closes #266