Why are SocketAddrV4/SocketAddrV6 based on low level sockaddr_in[6]?

[bascule]

@Nemo157

There was some brief discussion on this as part of

See also the previous rust-internals thread:

std::net::Ipv4Addr in core?

I'm an embedded developer so I don't have access to std. Could we move std::net::Ipv4 to core?

As noted in that thread and the linked RFC, avoiding dependence on libc::sockaddr would allow potentially moving these types to core, which would be nice for embedded applications.