Simple optimizations

I've made some very simple and trivial optimizations. In my tests (with @alcarvalho's branch, althrought I can't play the game on SWT because of some strange issue, I can see FPS on console), speed went up by 0.5 FPS on Pitfall.

* Optimized `*`, `/` and `**` to use shifts (`<<` and `>>`).
* Removed a block on `on_change` (blocks are quite slow in Ruby)