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/AI/Gormlak.hs

@@ -0,0 +1,39 @@
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+--------------------------------------------------------------------------------
+module Xanthous.AI.Gormlak () where
+--------------------------------------------------------------------------------
+import           Xanthous.Prelude hiding (lines)
+--------------------------------------------------------------------------------
+import           Data.Coerce
+import           Control.Monad.State
+--------------------------------------------------------------------------------
+import           Xanthous.Data (Positioned(..))
+import qualified Xanthous.Entities.Creature as Creature
+import           Xanthous.Entities.Creature (Creature)
+import qualified Xanthous.Entities.RawTypes as Raw
+import           Xanthous.Entities (Entity(..), Brain(..), brainVia)
+import           Xanthous.Game.State (entities, GameState)
+import           Xanthous.Data.EntityMap.Graphics (linesOfSight)
+--------------------------------------------------------------------------------
+
+stepGormlak :: MonadState GameState m => Positioned Creature -> m (Positioned Creature)
+stepGormlak (Positioned pos creature) = do
+  lines <- uses entities $ linesOfSight pos (Creature.visionRadius creature)
+  let newPos = fromMaybe pos
+               $ fmap fst
+               . headMay <=< tailMay <=< headMay
+               . sortOn (Down . length)
+               $ lines
+  pure $ Positioned newPos creature
+
+newtype GormlakBrain = GormlakBrain Creature
+
+instance Brain GormlakBrain where
+  step = fmap coerce . stepGormlak . coerce
+--------------------------------------------------------------------------------
+
+instance Brain Creature where step = brainVia GormlakBrain
+
+instance Entity Creature where
+  blocksVision _ = False
+  description = view $ Creature.creatureType . Raw.description