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/Item.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE StandaloneDeriving #-}
 --------------------------------------------------------------------------------
 module Xanthous.Entities.Item
   ( Item(..)
@@ -13,7 +14,14 @@ import           Data.Aeson.Generic.DerivingVia
 --------------------------------------------------------------------------------
 import           Xanthous.Entities.RawTypes hiding (Item, description)
 import qualified Xanthous.Entities.RawTypes as Raw
-import           Xanthous.Entities (Draw(..), Entity(..), DrawRawChar(..))
+import           Xanthous.Entities
+                 ( Draw(..)
+                 , Entity(..)
+                 , DrawRawChar(..)
+                 , Brain(..)
+                 , Brainless(..)
+                 , brainVia
+                 )
 --------------------------------------------------------------------------------
 
 data Item = Item
@@ -27,6 +35,9 @@ data Item = Item
                        Item
 makeLenses ''Item
 
+-- deriving via (Brainless Item) instance Brain Item
+instance Brain Item where step = brainVia Brainless
+
 instance Arbitrary Item where
   arbitrary = Item <$> arbitrary