add GameType.game_type which stores player and wall types, add GameUpdate.update function which takes a game type and returns a new game type, and refactor player/wall files, and gl-draw file, in light of these changes

fcore/game-update.sml

@@ -0,0 +1,10 @@
+structure GameUpdate =
+struct
+  fun update (game, input) =
+    let
+      val {player, walls, wallTree} = game
+      val player = Player.move (game, input)
+    in
+      {player = player, walls = walls, wallTree = wallTree}
+    end
+end