Track entity collision in the Entity class

Rather than having a single function in the Game.Lenses module for
determining what collision type if any an entity has, track it in the
Entity typeclass itself. This is both more extensible and a better
separation of concerns and gets rid of one of the two needs for a
circular import. Yay!

As part of this, I realized nothing was being done to prevent doors from
being placed on tiles that already had walls (since now that was
properly causing a collision!) so I've fixed that as well.

src/Xanthous/Generators/LevelContents.hs

@@ -45,6 +45,8 @@ randomDoors cells = do
     candidateCells = filter doorable $ Arr.indices cells
     subsetRange = (0.8 :: Double, 1.0)
     doorable (x, y) =
+      not (fromMaybe True $ cells ^? ix (x, y))
+      &&
       ( fromMaybe True $ cells ^? ix (x - 1, y) -- left
       , fromMaybe True $ cells ^? ix (x, y - 1) -- top
       , fromMaybe True $ cells ^? ix (x + 1, y) -- right