Migrate babysnake to retro_gamer's observation_function

BabySnake's tabular Q-learning exercise needs a small discrete state,
not the flattened board encoding retro_gamer normally produces. Define
get_state(game) in babysnake_env.py, returning a plain
(agent_x, agent_y, food_x, food_y) tuple, and point babysnake's
observation_function at it instead of declaring a character_set.
Disable the on-screen state overlay so it doesn't clutter the
terminal watch view.

babysnake/__init__.py

@@ -101,6 +101,7 @@ def create_game():
         },
         board_size=(bw, bh),
         framerate=6,
+        show_state=False,
     )
     forager.position = (randint(0, bw - 1), randint(0, bh - 1))
     food.relocate(game)