Fully seal ToSocketAddrs

[seanmonstar]

Currently the ToSocketAddrs trait is meant only for bounds, and all methods are supposed to be implementation details. This is done by putting them methods on "private" super trait. However, due to the way bounds on generics works, these methods are accidentally available to users.

fn bound<T: ToSocketAddrs>(t: T) {
    // But legal here, since T: ToSocketAddrs leaks the super trait methods
    t.to_socket_addrs();
}

fn main() {
    // Uncomment to see error
    //"127.0.0.1:0".to_socket_addrs();
    bound("127.0.0.1:0");
}

The solution is to define a private Internal argument type that must also be passed, so a user will see a compilation error when trying to call t.to_socket_addrs(), since they didn't call t.to_socket_addrs(Internal), and can't!

[taiki-e]

Probably FromStream has the same issue.