[Haskell-cafe] Generalizing "unlift" functions with monad-control

[Hiromi ISHII]

P.S. oops, I missed to attach the appropreate language extensions.
You have to specify following  exntensions to get my example works:

```haskell
{-# LANGUAGE ConstraintKinds, FlexibleContexts, FlexibleInstances   #-}
{-# LANGUAGE MultiParamTypeClasses, RankNTypes, ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies, TypeOperators                            #-}
```

-- Hiromi ISHII
konn.jinro at gmail.com