Proposal : ContT : minimal usage example of shift/reset

[original issue 81 by @ocramz]

Since delimited continuations are far from intuitive, I would like to add at least one usage example to their haddocks, something along these lines :

-- | demonstration of non-local control flow with a single shift/reset pair

--

-- λ> flip runState [] $ evalContT t1

-- (2,[2,0,1])

t1 :: ContT Int (State [Int]) Int

t1 = resetT $ do

  let

    x = 1 -- input

    cons w = lift $ modify (w :)

  r <- shiftT $ \k -> do

    cons x -- initial state uses the input

    let y = succ x -- compute a function of the input

    z <- lift $ k y -- delegate to the continuation k

    cons z -- mutate state with the return value of k

    pure y

  cons 0

  pure r