Proposal: ContT : add minimal usage example of delimited continuations

[original issue 80 by @ocramz]

Since shift/reset are not intuitive, I'd like to add at least one example to the ContT haddock, something along the lines of :

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

--

-- λ> run t1

-- ('b',"b_a")

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

t1 = resetT $ do

  let

    x = 'a' -- input

    cons w = lift $ modify (w :)

  r <- shiftT $ \k -> do

    cons x -- initial state uses the input

    let x&#39; = succ x -- compute a function of the input

    y <- lift $ k x&#39; -- delegate to the continuation k

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

    pure x&#39;

  cons &#39;_&#39;

  pure r