Yield until graph object update

[dphblox]

As part of the design of Eventuals (#4), Fusion is aiming to better support long-running imperative sequences embedded in a reactive system. As part of this, the process might wait for an object to change.

Right now, this would have to be implemented via polling:

scope:Eventual(function(use)
    repeat task.wait() until peek(state) == "foo"
end)

Ideally though, the change would be pushed to the imperative code:

scope:Eventual(function(use)
    repeat await(state) until peek(state) == "foo"
end)

Specific API surface TBD.