RFC: Make Take and Skip iterators DoubleEndedIterators when built from ExactSize iterators

Right now, the following does not work:

fn main() {
    let v = &[1, 2, 3, 4, 5];
    for x in v.iter().take(3).rev() {
        println!("{:}", x);
    }
}

test.rs:3:11: 3:33 error: type std::iter::Take<std::vec::Items<,<generic integer #4>>> does not implement any method in scope named rev
test.rs:3 for x in v.iter().take(3).rev() {

This would require adding an extra field to both of the iterators so I'm not sure if it's worth it.