Implement the start of creature AI

Add a Brain class, which determines for an entity the set of moves it
makes every step of the game, and begin to implement that for gormlaks.
The idea here is that every step of the game, a gormlak will move
towards the furthest-away wall it can see.

src/Xanthous/Data.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE RoleAnnotations #-}
 {-# LANGUAGE RecordWildCards #-}
 {-# LANGUAGE DeriveTraversable #-}
 {-# LANGUAGE DeriveFoldable #-}
@@ -74,6 +75,7 @@ data Positioned a where
   Positioned :: Position -> a -> Positioned a
   deriving stock (Show, Eq, Ord, Functor, Foldable, Traversable, Generic)
   deriving anyclass (CoArbitrary, Function)
+type role Positioned representational
 
 _Positioned :: Iso (Position, a) (Position, b) (Positioned a) (Positioned b)
 _Positioned = iso hither yon