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.

test/Xanthous/GameSpec.hs

@@ -46,6 +46,8 @@ test
     ]
   , testGroup "Saving the game"
     [ testProperty "forms a prism" $ isPrism saved
+    , testProperty "round-trips" $ \gs ->
+        loadGame (saveGame gs) === Just gs
     , testProperty "preserves the character ID" $ \gs ->
         let Just gs' = loadGame $ saveGame gs
         in gs' ^. character === gs ^. character