Implement Into_Iter for EdwardsPoint and RistrettoPoint

[CPerezz]

It would be nice to have an implementation that returns an iterator over the 4 coordinates of a RistrettoPoint or an EdwardsPoint.

Ex:

use zerocaf::edwards::EdwardsPoint;
use zerocaf::traits::Identity();
let identity = EdwardsPoint::Identity();
identity.into_iter().map(|coordinate| do_something_with_cooord(coordinate)).collect();