Fix user changes handling (#803)

* add failing test from @Johannes0021

* apply fix on update_positions

* apply fix on ColliderSet::iter_mut

* fix clippy..

* more complete test

* feat: refactor modified sets into a wrapper to avoid future mistakes

* chore: fix typos

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Co-authored-by: Sébastien Crozet <sebcrozet@dimforge.com>

src/geometry/mod.rs

@@ -182,6 +182,7 @@ impl ContactForceEvent {
 }
 
 pub(crate) use self::broad_phase::BroadPhaseProxyIndex;
+pub(crate) use self::collider_set::ModifiedColliders;
 pub(crate) use self::narrow_phase::ContactManifoldIndex;
 pub(crate) use parry::partitioning::Qbvh;
 pub use parry::shape::*;