Function iter_set::union

pub fn union<T, L, R>(
    a: L, 
    b: R
) -> impl Iterator<Item = T> where
    T: Ord,
    L: IntoIterator<Item = T>,
    R: IntoIterator<Item = T>, 

Take the union of two sets represented by sorted, deduplicated iterators.

If an elements is in both iterators, then only the one from a is yielded.

Time complexity: O(a.len() + b.len()).