Progressively reveal the map to the player

As the character walks around the map, progressively reveal the entities
on the map to them, using an algorithm based on well known
circle-rasterizing and line-rasterizing algorithms to calculate lines of
sight that are potentially obscured by walls.

test/Xanthous/Data/EntityMapSpec.hs

@@ -11,8 +11,12 @@ main :: IO ()
 main = defaultMain test
 
 test :: TestTree
-test = testGroup "Xanthous.Data.EntityMap"
+test = localOption (QuickCheckTests 20)
+  $ testGroup "Xanthous.Data.EntityMap"
   [ testBatch $ monoid @(EntityMap Int) mempty
+  , testGroup "Deduplicate"
+    [ testBatch $ monoid @(Deduplicate Int) mempty
+    ]
   , testGroup "Eq laws"
     [ testProperty "reflexivity" $ \(em :: EntityMap Int) ->
         em == em