Use existing monad infrastructure

[judah]

Haskeline's monadic interface has custom MonadIO and MonadTrans classes. It might be nice to use an existing framework like the mtl library; but we need theMonadException class which isn't in mtl.

Other implementations of MonadException:

• http://hackage.haskell.org/packages/archive/hscurses/1.3/doc/html/UI-HSCurses-MonadException.html
• http://hackage.haskell.org/trac/haskell-prime/ticket/110
• http://darcs.haskell.org/takusen/Control/Exception/MonadIO.hs

[anonymous]

Done on posix; the Win32 backend is probably broken but should be easy to fix.

[judah]

Done on Win32.