Implement saving+loading the game

Implement ToJSON and FromJSON for all of the various pieces of the game
state, and add a pair of functions saveGame/loadGame implementing a
prism to save the game as zlib-compressed JSON. To test this, there's
now Arbitrary, CoArbitrary, and Function instances for all the parts of
the game state - to get around circular imports with the concrete
entities this unfortunately is happening via orphan instances, plus an
hs-boot file to break a circular import that was just a little too hard
to remove by moving things around. Ugh.

src/Xanthous/Game/Lenses.hs

@@ -28,6 +28,7 @@ import           Xanthous.Entities.Character (Character, mkCharacter)
 import           Xanthous.Entities.Environment (Door, open)
 import           Xanthous.Entities.Item (Item)
 import           Xanthous.Entities.Creature (Creature)
+import           Xanthous.Entities.Entities ()
 --------------------------------------------------------------------------------
 
 getInitialState :: IO GameState