mechanical update

This just fits things with the more recent 4.0. It seemed fine with the following dimwitted tests, `digits` and `nonsense`:

    import Pipes
    import qualified Pipes.Prelude as P
    import Pipes.Random 
    import qualified System.Random as R


    rand :: Producer Int IO ()
    rand = do g <- lift (R.getStdGen)
              random g
    randR :: Int -> Int -> Producer Int IO ()
    randR n m = do g <- lift (R.getStdGen)
                   randomR (n,m) g
           
    consumer = P.take 100 >-> P.map show >-> P.stdout

    digits = run $ randR 0 9 >-> consumer
    nonsense = run $ rand >-> consumer