std: Stabilize FromStr implementations in std::net

The IP and socket address types all had `FromStr` implemented but the
implementations were not marked stable, nor was the error type returned ready to
be properly stabilized.

This commit marks the implementations of `FromStr` as stable and also renamed
the `ParseError` structure to `AddrParseError`. The error is now also an opaque
structure that cannot be constructed outside the standard library.

cc #22949
[breaking-change]

src/libstd/net/mod.rs

@@ -25,6 +25,7 @@ pub use self::ip::{Ipv4Addr, Ipv6Addr, Ipv6MulticastScope};
 pub use self::addr::{SocketAddr, SocketAddrV4, SocketAddrV6, ToSocketAddrs};
 pub use self::tcp::{TcpStream, TcpListener};
 pub use self::udp::UdpSocket;
+pub use self::parser::AddrParseError;
 
 mod ip;
 mod addr;