`monadCont` instance doesn't preserve cancellation behavior

[natefaubion]

I found this while working on a test for cancelling forkAll.

delay :: forall eff. Int -> Aff eff Unit
delay n = callCC \cont ->
  later' n (cont unit)

fork :: forall eff. Aff (console :: CONSOLE | eff) Unit
fork = do
  a <- forkAff (later' 10000 (Console.log "oops"))
  cancel a (error "bye")
  Console.log "Canceled a"
  b <- forkAff (do delay 10000
                   Console.log "oops")
  cancel b (error "bye")
  Console.log "Canceled b"

I would assume that the two formulations would be identical, but the callCC version does not cancel correctly. The first variation (using later' directly) gets cancelled correctly and prints "Canceled a". While the callCC variation waits 10 seconds, then get's cancelled, but does not print anything.