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/Data.hs

@@ -115,7 +115,7 @@ type Position = Position' Int
 
 instance Arbitrary a => Arbitrary (Position' a) where
   arbitrary = genericArbitrary
-  shrink = genericShrink
+  shrink (Position px py) = Position <$> shrink px <*> shrink py
 
 
 instance Num a => Semigroup (Position' a) where