Fix an injectivity issue with saving the game

Fix an injectivity issue with JSON-encoding the entity map that was
causing the game saving to not properly round-trip. As part of this,
there's a refactor to the internals of the entity map to use sets
instead of vectors, which should also get us a nice perf boost.

src/Xanthous/Entities/Character.hs

@@ -39,7 +39,7 @@ data Character = Character
   , _characterHitpoints' :: !Double
   , _speed :: TicksPerTile
   }
-  deriving stock (Show, Eq, Generic)
+  deriving stock (Show, Eq, Ord, Generic)
   deriving anyclass (NFData, CoArbitrary, Function)
   deriving (ToJSON, FromJSON)
        via WithOptions '[ FieldLabelModifier '[Drop 1] ]