main, physics: added benchmarking mode

Is stack faster than heap? How much? Is it worth substituting 2 assignments and one division for no assignments but 3 divisions? Now you can see whether what you have is snake oil. The timer itself is less than reliable because of the 100 microsecond sleep(due to novid) but if whatever you're running(and what you're running it on) can clock at least a single timestep per second you'll have some idea of the performance difference.