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/Entities/Environment.hs

@@ -13,7 +13,15 @@ import Brick (str)
 import Brick.Widgets.Border.Style (unicode)
 import Brick.Types (Edges(..))
 --------------------------------------------------------------------------------
-import Xanthous.Entities (Draw(..), entityIs, Entity(..), SomeEntity)
+import Xanthous.Entities
+       ( Draw(..)
+       , entityIs
+       , Entity(..)
+       , SomeEntity
+       , Brain(..)
+       , Brainless(..)
+       , brainVia
+       )
 import Xanthous.Entities.Draw.Util
 import Xanthous.Data
 --------------------------------------------------------------------------------
@@ -22,6 +30,9 @@ data Wall = Wall
   deriving stock (Show, Eq, Ord, Generic, Enum)
   deriving anyclass (CoArbitrary, Function)
 
+-- deriving via Brainless Wall instance Brain Wall
+instance Brain Wall where step = brainVia Brainless
+
 instance Entity Wall where
   blocksVision _ = True
   description _ = "a wall"
@@ -64,6 +75,9 @@ instance Draw Door where
       horizDoor = '␣'
       vertDoor = '['
 
+-- deriving via Brainless Door instance Brain Door
+instance Brain Door where step = brainVia Brainless
+
 instance Entity Door where
   blocksVision = not . view open
   description _ = "a door"