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/Entities/Character.hs

@@ -40,7 +40,7 @@ data Character = Character
   , _speed :: TicksPerTile
   }
   deriving stock (Show, Eq, Generic)
-  deriving anyclass (CoArbitrary, Function)
+  deriving anyclass (NFData, CoArbitrary, Function)
   deriving (ToJSON, FromJSON)
        via WithOptions '[ FieldLabelModifier '[Drop 1] ]
            Character