unify `Compose` and `Klesli`

[marcosh]

Currently we have both the Compose and Kleisli constructors. They are fairly similar, but both needed.

It would be nice to be able to unify them. This could depend on understanding a meaningful way to emit multiple outputs from the same machine (meaning that StateMachine a b could emit multiple outputs of type b). The main issue I see with this is understanding the return type of Parallel; if we just zip the results of two machines, we might loose stuff