Make cloning rigid-bodies and colliders more idiomatic.

Fix #53

src/geometry/collider_set.rs

@@ -59,6 +59,10 @@ impl ColliderSet {
         parent_handle: RigidBodyHandle,
         bodies: &mut RigidBodySet,
     ) -> ColliderHandle {
+        // Make sure the internal links are reset, they may not be
+        // if this rigid-body was obtained by cloning another one.
+        coll.reset_internal_links();
+
         coll.parent = parent_handle;
         let parent = bodies
             .get_mut_internal(parent_handle)