• (waiting-stream-check w success-cont failure-cont)
Take a waiting stream, a success continuation, and a failure continuation.
 If we don't find any ready suspended streams, invoke the failure continuation.
 If we find a ready suspended stream calculate the remainder of the waiting
 stream. If we've reached the fixpoint just call the thunk of the suspended
 stream, otherwise call mplus on the result of the thunk and the remainder
 of the waiting stream. Pass this result to the success contination.