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/Command.hs

@@ -17,6 +17,7 @@ data Command
   | Open
   | Wait
   | Eat
+  | Save
 
     -- | TODO replace with `:` commands
   | ToggleRevealAll
@@ -30,6 +31,7 @@ commandFromKey (KChar ',') [] = Just PickUp
 commandFromKey (KChar 'o') [] = Just Open
 commandFromKey (KChar 'e') [] = Just Eat
 commandFromKey (KChar 'r') [MMeta] = Just ToggleRevealAll
+commandFromKey (KChar 'S') [] = Just Save
 commandFromKey _ _ = Nothing
 
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