Replaced collision mechanism with a simpler mechanism.

This PR is an attempt at simplifying the collision mechanism. I'm not entirely sure that this is an improvement. Although much simpler, there are some consequences:

* Collision code is now part of the `Unit` class. This doesn't feel like the appropriate place. It introduces a coupling between the concept of a `Unit` and collisions amongst other `Units`.
* Extending collision behaviour becomes a lot harder. Adding a new implementation of `Unit` to the game (like fruit, super pellets, or miss JPacman), the collision code for existing `Unit` implementations will need to be extended or overridden somehow. This is far easier to do with the collision map.

Thoughts?