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.
In this commit, we add two new fundamental data structures: Future[T]
and Promise[T].
A future is a response that might be ready at some point in the future.
This is already a common pattern in Go, we just make a type safe wrapper
around the typical operations: block w/ a timeout, add a call back for
execution, pipeline the response to a new future.
A promise is an intent to complete a future. Typically the caller
receives the future, and the callee is able to complete the future using
a promise.
Olaoluwa Osuntokun