get_next and get_prev on OrdMap/Set don't appear to work beyond a single node

[rphmeier]

Here's a minimal reproduction:

    let COUNT: usize = 65;

    let mut values = (0..65).map(|i| (i + 1) * 2).collect::<Vec<_>>();

    let set = values.iter().cloned().collect::<im::ordset::OrdSet<usize>>();
    for value in &values {
        let next = *value + 1;
        assert_eq!(set.get_prev(&next), Some(value));

        let prev = *value - 1;
        assert_eq!(set.get_next(&prev), Some(value));
    } 

Note that if you set count to 64 or less, this works.