Be able to perform unhandled effects

[dinosaure]

Currently, this PR tries to fix the issue #10: what Miou should do when we catch another effect. I did a try with this code:

type _ Effect.t += Foo : unit Effect.t

let prgm () =
  let prm = Miou.call_cc @@ fun () -> Effect.perform Foo in
  Miou.await_exn prm

let handler fn v =
  let open Effect.Deep in
  let retc = Fun.id in
  let exnc = raise in
  let effc : type c. c Effect.t -> ((c, 'a) continuation -> 'b) option =
    function
    | Foo -> Some (fun k -> continue k ())
    | _ -> None
  in
  match_with fn v { retc; exnc; effc }

let () = handler (fun () -> Miou.run prgm) ()

So we can plug an other handler on top of Miou which handle our effects. However, we keep the same rule about our "quanta": a program is suspended when it emits one effect (even if the effect is not handled by Miou). The current issue is about parallel task. Indeed, the handler is only well know by dom0 but if we use Miou.call, we will get an exception. We can figure out about an optional argument passed by Miou.run which will "protect" tasks with our handler on other domains.

/cc @patricoferris (who asking) @hannesm (who is interested by that)

[dinosaure]

It seems that nobody has an strong opinion about that, let's merge and see what happens with such interface.