Make Entities iterable

[leudz]

To make it possible the first entity added in the removed linked list must have its index changed to any value (usize::MAX for example).
We can then use the fact that if entities[index].index() == index this entity is alive.

[djahandarie]

I found myself wanting this when trying to build a generalized viewer of my entire World for debugging/development purposes. (Just ended up manually iterating over all the different entities types available.)

[leudz]

I don't know any better workaround but that's pretty horrible 😄