In this commit, we add the actual Actor implementation. We define a
series of types and interfaces, that in concert, describe our actor. An
actor has some ID, a reference (used to send messages to it), and also a
set of defined messages that it'll accept.
An actor can be implemented using a simple function if it's stateless.
Otherwise, a struct can implement the Receive method, and handle its
internal message passing and state that way.
