Move the platform independent part of net into core

[bascule]

Related:

RFC: Move `std::net::IpAddr` types into `core::net`.

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?

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

A port is always 16 bits (2 bytes). An IPv4 address is 4 bytes. This means the optimal size for SocketAddrV4 is 6 bytes (8 bytes with alignment 4 for Ipv4Addr). However, size_of::<SocketAddrV4>() == 16 (on Linux). This is 2x the expected size Hmm.. Yes, this is because the implementation is: pub struct SocketAddrV4 { inner: c::sockaddr_in, } And sockaddr_in has more data than just the IP and port. It's also not (currently) possible to make the new constructors for these sock…