Improve distant object panics in broad phase

[aristaeus]

This improves the situation in #77:

• The region key has been replaced by an i64 in the f64 version of the library, increasing the range before panics occur.
• Asserts have been added to panic when creating the region key, with a comment explaining the issue to users still affected.

[Vrixyz]

I updated the branch here: https://github.com/Vrixyz/rapier/tree/broad_phase_panic_fix-updated

[Vrixyz]

Thanks!