Function update_and_norm 

fn update_and_norm(
    y: &mut [Complex64],
    x: &[Complex64],
    parts: &[usize],
    f: impl Fn(&mut Complex64, Complex64) + Sync + Send,
) -> f64

f applied elementwise to y (paired with x), then cnorm_parts(y), in one pass: the same values as the two passes, part by part.