wire: use core::net types for IP addresses.

[Dirbaio]

• wire: move prefix_len impl down to v4 and v6 addrs.
• wire: use core::net::Ipv4Addr as the ipv4 address type.
• Bump MSRV to 1.80.

[thvdveld]

I opened rust-lang/libs-team#447, as requested in rust-lang/rust#130629 (comment).

[Dirbaio]

this is ready to review.

I've "fixed" the unstable_name_collisions warnings by renaming the affected methods to x_. it's not pretty but it's the most reasonable solution I think. (note it doesn't affect the public API, the extension trait is private).

I still think we should merge this, moving to the stdlib's ip addr types is worth it even if it requires carrying a few warts internally.