impl AsIpv4Addrs refactoring

[keepsimple1]

When trying to use the latest code in main, I found that it missed impl AsIpv4Addrs trait for &String, which I used with previous versions.

To support AsIpv4Addrs with reference types and avoid redundancy, this diff changes the rest of impls to use reference types.

This is a follow-up to PR #21 . @indietyp any comments/thoughts?

[indietyp]

I must've deleted the implementation, but there was an original implementation. (f2627ac#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R1370)

Tho, it would have been:

impl<T: AsIpv4Addrs> for &T {
    fn as_ipv4_addrs(&self) {
       T::as_ipv4_addrs(self)
    }
}

or something similar.

[indietyp]

this would enable usage for owned and borrowed types

[indietyp]

I just checked, I must've removed the implementation by accident in 70443c12309a5c0e014696f9a0795dfcffea038d.

[keepsimple1]

Thanks for bringing that up. That's a better approach! I updated the diff.