Iterator API is still too slow

[robertlong]

HECS performs worse than other ECS frameworks in the TowerDefenceECS project. In Firefox, Perf.html shows around 38% of the time being spent in next() so there is clearly some work to be done.

[brianpeiris]

FYI, I'm going to remove HECS from TowerDefenseECS for now. I ended up optimizing my three.js usage which allowed me to increase the number of entities, but HECS does worse since there are more iterations in my loops now. Don't want to misrepresent HECS until it's ready.