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/Util/QuickCheck.hs

@@ -0,0 +1,28 @@
+module Xanthous.Util.QuickCheck
+  ( FunctionShow(..)
+  , functionJSON
+  , FunctionJSON(..)
+  ) where
+--------------------------------------------------------------------------------
+import Xanthous.Prelude
+import Test.QuickCheck
+import Test.QuickCheck.Function
+import Test.QuickCheck.Instances.ByteString ()
+import Data.Aeson
+import Data.Coerce
+--------------------------------------------------------------------------------
+
+newtype FunctionShow a = FunctionShow a
+  deriving newtype (Show, Read)
+
+instance (Show a, Read a) => Function (FunctionShow a) where
+  function = functionShow
+
+functionJSON :: (ToJSON a, FromJSON a) => (a -> c) -> a :-> c
+functionJSON = functionMap encode (headEx . decode)
+
+newtype FunctionJSON a = FunctionJSON a
+  deriving newtype (ToJSON, FromJSON)
+
+instance (ToJSON a, FromJSON a) => Function (FunctionJSON a) where
+  function = functionJSON