Use sparse set components for fragmented_iter for Bevy

[alice-i-cecile]

As discussed in the 0.5 news release, sparse set component storage was designed to make fragmented iteration faster, precisely the use case being tested in the fragmented_iter benchmark.

[cart]

Yup other ECS-es currently use "fast path" apis, such as SoA insertion and for_each iterators (despite other apis being "more idiomatic" or "recommended"). I think it is fair play at this point to do the same for Bevy.

However this is my current take on ecs_bench_suite policy/labeling in general: #18 (comment)

The current approach biases toward the wrong behaviors and makes it harder for developers to make an informed decision when picking an ECS.