Add debug command to reveal the game

Add a (debug) command to reveal all tiles on the game regardless of the
character's vision, which'll make it easier to debug creature's behavior
while they're not visible.

src/Xanthous/Game.hs

@@ -21,6 +21,11 @@ module Xanthous.Game
 
     -- * App monad
   , AppT(..)
+
+    -- * Debug State
+  , DebugState(..)
+  , debugState
+  , allRevealed
   ) where
 --------------------------------------------------------------------------------
 import Xanthous.Game.State