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)
Neither @aturon or I wanted to keep this per se, but the only real alternative we could think of is requiring `fn main() -> int` of some form which I personally see as untenable. Do you have a suggestion for how you'd rather see this instead?