std: Deprecate the std::old_io::net primitives

The `std::net` primitives should be ready for use now and as a result the old
ones are now deprecated and slated for removal. Most TCP/UDP functionality is
now available through `std::net` but the `std::old_io::net::pipe` module is
removed entirely from the standard library.

Unix socket funtionality can be found in sfackler's [`unix_socket`][unix] crate
and there is currently no replacement for named pipes on Windows.

[unix]: https://crates.io/crates/unix_socket

[breaking-change]

src/libstd/old_io/net/pipe.rs

@@ -19,6 +19,12 @@
 //! instances as clients.
 
 #![allow(missing_docs)]
+#![deprecated(since = "1.0.0",
+              reason = "will be removed to be reintroduced at a later date; \
+                        in the meantime consider using the `unix_socket` crate \
+                        for unix sockets; there is currently no replacement \
+                        for named pipes")]
+#![unstable(feature = "old_io")]
 
 use prelude::v1::*;