This marks a function as the method that should be used to create the actor. It emits a method
which returns a tuple of the address to the actor and its ActorManager.
The main item of the crate. This is a proc macro used as an attribute on the actor struct
definition, Actor implementation, and on an impl block in which the handler functions are used.
This marks a function as the method that should be used to create and spawn the actor. It must
return the type of the actor, either as Self or by its name.