Re-addition of std::net::IpAddr #988
Description
(Firstly, I saw the discussion around its removal in #923, but that decision appears to have been made before SocketAddr was decided to have an enum structure like that of IpAddr. I'm thinking maybe it was an oversight that IpAddr was left out after that decision. In case it wasn't an oversight, here's my argument in favor of re-introducing IpAddr.)
I propose that the type IpAddr, as an enum with V4(Ipv4Addr) and V6(Ipv6Addr) variants should be re-added to std::net. In my experience, it is often necessary to express an IP address that is either IPv4 or IPv6 without the address being associated with a socket and, therefore, it not having an associated port value. For example, when a server retains the remote addresses of past clients. In such cases, it is not intuitive to store such an address as a SocketAddr instance. Also, the re-addition would mirror the SocketAddr enum, which itself has V4 and V6 variants containing structures to the specific types.
Additionally, I propose that the std::net::LookupHost type should be reverted to yield IpAddr types rather than SocketAddr as this also permits dealing with hostnames in cases where they are not directly related to any socket operations. It should be noted that the ToSocketAddrs trait is already widely used within std::net to yield a series of SocketAddr instances and keeping lookup_host as-is would constitute a duplication of this functionality.
With these changes, I think it would also be helpful to add a convenience method for creating SocketAddr instances; e.g. fn new(ip: &IpAddr, port: u16) -> SocketAddr.