Linking legion entities with rapier2d colliders.

Hello,

I'm trying to combine legion with rapier2d for physics calculation and 2d . So if I make a query in rapier2d and get a certain rigid body or collider I want to be able to get the legion entity.

For this reason rapier2d provides a u128 userdata linked to each collider, however, the internal legion entity id (NonZeroU64) is private and can therefore not be used.

Would it be possible to provide a way to get the internal entity id and create a new entity using a known id, like:

```
collider.set_userdata(entity.get_id());
...
let entity = World::get_entity(collider.get_userdata() as NonZeroU64).unwrap();
```

Or do you even know of a a better solution? The only possible alternative that comes to my mind is building a separate u128 id and mapping it to legions entities. However, this is a lot of boilerplate and effectively managing ids on ids...

Thanks in advance and thanks for this incredible ECS!