Implement FromStr for IpAddr and SocketAddr

FromStr implemented from scratch.

It is overengineered a bit, however.

Old implementation handles errors by fail!()-ing. And it has bugs, like it accepts `127.0.0.1::127.0.0.1` as IPv6 address, and does not handle all ipv4-in-ipv6 schemes. So I decided to implement parser from scratch.