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.

src/Xanthous/Entities/Character.hs

@@ -14,6 +14,9 @@ data Character = Character
   deriving anyclass (CoArbitrary, Function)
   deriving Draw via (DrawCharacter "@" Character)
 
+instance Entity Character where
+  blocksVision _ = False
+
 instance Arbitrary Character where
   arbitrary = pure Character