Very few entities actually hard collide, so store them in their own
entity slices and provide a special getEntites type call just for them.
This reduces entity collision checking impact (in my testing) by 25%
for crammed entities (shove 130 cows into an 8x6 area in one chunk).
Less crammed entities are likely to show significantly less benefit.
Effectively, this patch optimises crammed entity situations.
