std: Rename io to old_io

In preparation for the I/O rejuvination of the standard library, this commit
renames the current `io` module to `old_io` in order to make room for the new
I/O modules. It is expected that the I/O RFCs will land incrementally over time
instead of all at once, and this provides a fresh clean path for new modules to
enter into as well as guaranteeing that all old infrastructure will remain in
place for some time.

As each `old_io` module is replaced it will be deprecated in-place for new
structures in `std::{io, fs, net}` (as appropriate).

This commit does *not* leave a reexport of `old_io as io` as the deprecation
lint does not currently warn on this form of use. This is quite a large breaking
change for all imports in existing code, but all functionality is retained
precisely as-is and path statements simply need to be renamed from `io` to
`old_io`.

[breaking-change]

src/libstd/lib.rs

@@ -79,7 +79,8 @@
 //! memory types, including [`atomic`](sync/atomic/index.html).
 //!
 //! Common types of I/O, including files, TCP, UDP, pipes, Unix domain sockets,
-//! timers, and process spawning, are defined in the [`io`](io/index.html) module.
+//! timers, and process spawning, are defined in the
+//! [`old_io`](old_io/index.html) module.
 //!
 //! Rust's I/O and concurrency depends on a small runtime interface
 //! that lives, along with its support code, in mod [`rt`](rt/index.html).
@@ -239,7 +240,7 @@ pub mod thread_local;
 pub mod dynamic_lib;
 pub mod ffi;
 pub mod fmt;
-pub mod io;
+pub mod old_io;
 pub mod os;
 pub mod path;
 pub mod rand;
@@ -284,7 +285,7 @@ mod std {
     pub use sync; // used for select!()
     pub use error; // used for try!()
     pub use fmt; // used for any formatting strings
-    pub use io; // used for println!()
+    pub use old_io; // used for println!()
     pub use option; // used for bitflags!{}
     pub use rt; // used for panic!()
     pub use vec; // used for vec![]