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)
Since Rust is moving to detached threads, this is an anti-pattern. It encourages a racy way of setting the exit status rather than simply returning it from main and encouraging it to be bubbled up sanely. It doesn't make sense to have arbitrary tasks clobbering the exit status at any point, and it adds unnecessary runtime infrastructure around `main`.