Implement sprite pooling

[spmurrayzzz]

Presently, all the asteroid sprites are created on-demand in perpetuity. This is problematic for frame rates in the long run.

Alternative here would be to create a finite amount of sprites up front and recycle them as needed.

[spmurrayzzz]

Added this functionality for laser and starfield sprites, but still need a game plan for asteroids (and future enemy entities).

For the time being, asteroids are removed from the display group on out of bounds events (which helps keep GC ticks small), but we're still creating them in a timed loop which is not performant.