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.

test/Xanthous/GameSpec.hs

@@ -44,4 +44,10 @@ test
           (oextend f . oextend g) mh === oextend (f . oextend g) mh
       ]
     ]
+  , testGroup "Saving the game"
+    [ testProperty "forms a prism" $ isPrism saved
+    , testProperty "preserves the character ID" $ \gs ->
+        let Just gs' = loadGame $ saveGame gs
+        in gs' ^. character === gs ^. character
+    ]
   ]