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.
