Convert collision detection to quadtree

[devstopfix]

See quadtree

Algorithm:

• build a quadtree of all rocks, bullets and ships
• get a list of collisions - each is a term {:bullet 1 :ship 12}
• filter duplicate events - currently a single bullet could hit a ship and a rock
• send events into the game

[devstopfix]

Erlang has a quadtree: https://github.com/g-andrade/erlquad

[devstopfix]

Collision detection takes less than 1ms at present, so not so vital