This makes it easy to write custom filters based on the handles.
For instance, a user can add pairs of `ColliderHandle`s into a `HashSet` and
do a lookup in a `ContactPairFilter` to easily (if maybe not efficiently)
disable collisions between pairs of colliders.
The same could be accomplished by using the `user_data: u128` field of the
`Collider`, but it would be less ergonomic, and also with this PR that
`user_data` space can be saved for more important things.
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