* Implement collision detection
* Minor cleanups.
* Add laser/player collisions
* Add laser collision with bullets and fix fire button repeating
* Add basic shield collisions
* Refactor collision and debug
* Simplify collision and debug by not tracking laser indices
- We don't care about which laser collided, because they get destroyed immediately.
* Don't track laser indicies against bullets either
* DRY and docs
* Adjust the fleet bounding box as invaders are shot
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