fn main() {
    let b = vec![0u32, 1, 2, 3];
    let a = b.into_iter()
             .map(|x: u32| x.to_ne_bytes())
             .flat_map(|x: [u8; 4]| {
                let [x, y, z, w] = x;
                use std::iter::once;
                once(x).chain(once(y)).chain(once(z)).chain(once(w))
                })
             .collect::<Vec<u8>>();
}