std: Add a net module for TCP/UDP

This commit is an implementation of [RFC 807][rfc] which adds a `std::net`
module for basic neworking based on top of `std::io`. This module serves as a
replacement for the `std::old_io::net` module and networking primitives in
`old_io`.

[rfc]: fillmein

The major focus of this redesign is to cut back on the level of abstraction to
the point that each of the networking types is just a bare socket. To this end
functionality such as timeouts and cloning has been removed (although cloning
can be done through `duplicate`, it may just yield an error).

With this `net` module comes a new implementation of `SocketAddr` and `IpAddr`.
This work is entirely based on #20785 and the only changes were to alter the
in-memory representation to match the `libc`-expected variants and to move from
public fields to accessors.

src/libstd/lib.rs

@@ -250,6 +250,7 @@ pub mod ffi;
 pub mod old_io;
 pub mod io;
 pub mod fs;
+pub mod net;
 pub mod os;
 pub mod env;
 pub mod path;