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/Test/Prelude.hs

@@ -13,6 +13,7 @@ import Test.Tasty.QuickCheck
 import Test.Tasty.HUnit
 import Test.QuickCheck.Classes
 import Test.QuickCheck.Checkers (TestBatch)
+import Test.QuickCheck.Instances.ByteString ()
 
 testBatch :: TestBatch -> TestTree
 testBatch (name, tests) = testGroup name $ uncurry testProperty <$> tests