Implement .range_index() for RedBlackTreeMap/RedBlackTreeSet

[liangyongrui]

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Is it possible to search RedBlackTreeMap by index?

like this

[orium]

Currently it's not possible. To have that with O(lg(n)) we would need to add some extra information to each node (the number of elements to the left).

I'm not sure if there is enough demand for this to spend and extra usize per node.