Function max_set_impl

pub fn max_set_impl<I, K, F, Compare>(
    it: I,
    key_for: F,
    compare: Compare,
) -> Vec<I::Item>
where
    I: Iterator,
    F: FnMut(&I::Item) -> K,
    Compare: FnMut(&I::Item, &I::Item, &K, &K) -> Ordering,

Implementation guts for ax_set, max_set_by, and max_set_by_key.