tilegrid,grid,graph: General clean up. This commit breaks the world :) Sorry about that.

As of now I don't understand the error message from SML when running demo.sml.
What do you guys think?

Error- in 'demo.sml', line 9.
Type error in function application.
   Function: Pathfinder.aStarGraph :
      Grid.grid -> (Pathfinder.Color * int * int * (int * int)) Graph.graph
   Argument: grid : Grid.grid
   Reason: Can't unify Grid.grid with Grid.grid (Different type constructors)
Found near Pathfinder.aStarGraph grid
Error trying to use the file: 'demo.sml'