Add convenience methods to IpAddr

[canndrew]

IpAddr should have at least the following methods:

/// Returns `true` if `self` is one of the two unspecified addresses
fn is_unspecified(&self) -> bool;

/// Returns `true` if `self` is a loopback address such as `127.0.0.1` or `::1`
fn is_loopback(&self) -> bool;

Edit: is_multicast and to_ipv6_{compatible,mapped} would make sense aswell although I have no need for them.

[canndrew]

Another thing I'd like is the ability to quickly create localhost and/or unspecified addresses. I sometimes find myself writing SocketAddr::V4(SocketAddrV4::new(Ipv4Addr::new(127, 0, 0, 1), port)) which is a very long way to say "localhost:port". A shorter version (when you can use ToSocketAddrs) is to write ("127.0.0.1", port) but then you're unnecessarily parsing a string every time the code runs.

Could we also add the following methods to the standard library?

fn Ipv4Addr::localhost() -> Ipv4Addr;
fn Ipv6Addr::localhost() -> Ipv6Addr;
fn IpAddr::localhost_v4() -> IpAddr;
fn IpAddr::localhost_v6() -> IpAddr;
fn SocketAddrV4::localhost(port: u16) -> SocketAddrV4;
fn SocketAddrV6::localhost(port: u16) - >SocketAddrV6;
fn SocketAddr::localhost_v4(port: u16) -> SocketAddr;
fn SocketAddr::localhost_v6(port: u16) -> SocketAddr;

And similarly for unspecified addresses?

[Anachron]

I suggest to start here: https://github.com/rust-lang/rfcs#before-creating-an-rfc
And follow the whole process. This RFC does not meet any standards or form :/

[canndrew]

Well it's not really an RFC, just a couple of ideas. I could move them into an RFC if people think it's worth it.

[canndrew]

This is fixed in rust-lang/rust#34694