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/Game/Arbitrary.hs

@@ -0,0 +1,27 @@
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+{-# LANGUAGE RecordWildCards #-}
+--------------------------------------------------------------------------------
+module Xanthous.Game.Arbitrary where
+--------------------------------------------------------------------------------
+import           Xanthous.Prelude
+--------------------------------------------------------------------------------
+import           Test.QuickCheck
+import           System.Random
+--------------------------------------------------------------------------------
+import           Xanthous.Game.State
+import           Xanthous.Entities.Arbitrary ()
+import           Xanthous.Entities.Character
+import qualified Xanthous.Data.EntityMap as EntityMap
+--------------------------------------------------------------------------------
+
+instance Arbitrary GameState where
+  arbitrary = do
+    char <- arbitrary @Character
+    charPos <- arbitrary
+    _messageHistory <- arbitrary
+    (_characterEntityID, _entities) <- arbitrary <&>
+      EntityMap.insertAtReturningID charPos (SomeEntity char)
+    _revealedPositions <- fmap setFromList . sublistOf $ EntityMap.positions _entities
+    _randomGen <- mkStdGen <$> arbitrary
+    let _promptState = NoPrompt -- TODO
+    pure $ GameState {..}