RFC: io and os reform

This RFC proposes a significant redesign of the `std::io` and `std::os` modules
in preparation for API stabilization. The specific problems addressed by the
redesign are given in the Problems section below, and the key ideas of the
design are given in Vision for IO.

[Rendered](https://github.com/aturon/rfcs/blob/io-os-reform/text/0000-io-os-reform.md)
I don't  think `set_exit_status(0)` is "more racy" than `stdout.write_all(b"Hello world")`.  Programmers will have to use sort of mutex to guard I/O states anyways.

That said, it would be nice if Rust supported returning an exit status code from `main()`.