Game removes bullets on its own, not bullet itself

Game checks for bullet collisions in its checkCollision method,
removing collided bullets on its own. Bullet no longer needs a ref to
the game for that. (They do still need a ref to game to remove
themselves after their lifetime; will fix that later.)
Asteroid children values added to fieldValue

Means that (oops!) the field won't get overpopulated anymore.

Also bullets use a timer to determine when they die.