Refactor game state computation

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`.

tetris.cabal

@@ -19,17 +19,17 @@ library
   build-depends:       base >= 4.7 && < 5
                      , brick
                      , containers
-                     , microlens
-                     , microlens-th
+                     , lens
                      , linear
                      , random
+                     , transformers
                      , vty
   default-language:    Haskell2010
 
 executable tetris
   hs-source-dirs:      app
   main-is:             Main.hs
-  ghc-options:         -threaded -rtsopts -with-rtsopts=-N
+  ghc-options:         -Wall -threaded -rtsopts -with-rtsopts=-N
   build-depends:       base >= 4.7 && < 5
                      , directory
                      , filepath