convenience: Iterator improvements

[vorner]

Hello

I'm reading through the code a bit. I've noticed few places for possible improvements around iterators:

• There's an iterator support for Ipv4Network as well as Ipv6Network. But there's not one for IpNetwork. I think providing a wrapper iterator would be quite trivial.
• The iterator types may benefit from some derived traits (eg. #[derive(Debug, Copy, Clone)].
• Oftentimes, if there's a .iter() method for a type, the type-reference implements IntoIterator. Eg. impl IntoIterator for &Ipv4Network.

If these would be considered reasonable additions, I'd volunteer to send the pull request.

[achanda]

A pull request will be welcome, thanks!