Implement `DoubleEndedIterator` for network iterators

[gammelalf]

It would be cool if it could iterate over a network in reverse order.

The most flexible solution would be implementing DoubleEndedIterator from the standard library for IpNetworkIterator, Ipv4NetworkIterator and Ipv6NetworkIterator

[gammelalf]

I just discovered that the std's Ipv4Addr implements Step which means it is compatible with ranges.
So my current solution is (net_v4.network()..=net_v4.broadcast()).rev().