[WIP] Bind Observables to a particular Scheduler

Per #8, similar to RxHaskell's [`Signal`](https://github.com/jspahrsummers/RxHaskell/blob/c6a673a799dd38ff0b2f7631e07a65500dd8baf0/Signal.hs#L33-L35) type. `Promise` then becomes something like the [`ScheduledIO`](https://github.com/jspahrsummers/RxHaskell/blob/c6a673a799dd38ff0b2f7631e07a65500dd8baf0/Scheduler/Internal.hs#L22-L24) monad.

Is this actually a good idea? I don't know.