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/EntitiesSpec.hs

@@ -0,0 +1,20 @@
+--------------------------------------------------------------------------------
+module Xanthous.EntitiesSpec where
+--------------------------------------------------------------------------------
+import           Test.Prelude
+--------------------------------------------------------------------------------
+import qualified Data.Aeson as JSON
+--------------------------------------------------------------------------------
+import           Xanthous.Entities
+--------------------------------------------------------------------------------
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Entities"
+  [ testGroup "EntityChar"
+    [ testProperty "JSON round-trip" $ \(ec :: EntityChar) ->
+        JSON.decode (JSON.encode ec) === Just ec
+    ]
+  ]