Cleaning up the Tetris module, mainly to favor a MonadState style over
explicit `Game -> Game` functions, which were a little awkward. This also
led naturally to explicitly isolating `IO`, by having pure state modifiers
with types like `Tetris a ~ forall m. StateT Game m a` being executed
with `m ~ Identity`, and those few modifiers that need IO specified by
`TetrisT a ~ StateT Game IO a`.
