Slab::drain() should return the indices of the elements

[tomaka]

When calling into_iter(), the Slab returns an iterator that yields (usize, T).

But when calling drain(), only Ts are returned, and the indices of the elements are lost.

This is an inconsistency in the API, but also knowing these indices is necessary for my use case.
I'm currently bypassing this with mem::replace(&mut slab, Slab::new()).into_iter().

[Darksonn]

That makes sense to me.