[hangman] Recreated hanfman game using the state monad

Observations:

- Code didn't get smaller, which was unexpected. It's possible that I'm
  still doing things the hard way.

- Was able to completely seperate game logic from IO. Could now take the
  HangmanM module and use it in, say, a web application or something.

- Monadic hangman is encouraging, because once I understood the state
  monad, it was easy to write. More like what I expected. Feels more
  like it properly relies on the types.