Remove reference to runner in asteroid

The asteroid is now destroyed in the next tick, instead of delaying
it to the tick after the next.
Remove unused parameter other in asteroid.destroy()

With the parameter, it looked like the method was intended to destroy
the 'other' asteroid only. Actually, the method destroys the current
asteroid and all asteroid in contact (transitively).
Merge branch 'asteroid' of github.com:eric-brechemier/coquette into eric-brechemier-asteroid
v. 0.2.4