Investigate performance overhead

[Vrixyz]

• With https://github.com/dimforge/bevy_rapier/pull/551, we noticed significant overhead within rapier's step function, which is not visible in rapier's benchmarks.

The performance bottleneck seems to some from contacts, which should probably not appear so often and stabilize quickly. An in-depth analysis had not been done yet, but the linked pull request adds useful reproduction as well as some instructions in readme.