ActorInit ergonomics

[JohnMurray]

Construction Boilerplate

The current usage of ActorInit in user code requires implementing a trait separate from the main actor trait. This is kinda bleh. It'd be great if we could unify this either through rewordking the Actor trait, or through some macro magic.

Ideally the overhead for creating the "constructor" for the actor is no more than a method.

No-Op Construction

It's not possible to have a no-arg (()) constructor. This should be possible, somehow, and ideally without requiring much/any code to do it.