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.

package.yaml

@@ -41,6 +41,7 @@ dependencies:
 - MonadRandom
 - mtl
 - optparse-applicative
+- parallel
 - random
 - random-fu
 - random-extras
@@ -97,6 +98,7 @@ executable:
   - -threaded
   - -rtsopts
   - -with-rtsopts=-N
+  - -O2
 
 tests:
   test:
@@ -106,6 +108,7 @@ tests:
     - -threaded
     - -rtsopts
     - -with-rtsopts=-N
+    - -O0
     dependencies:
     - xanthous
     - tasty