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)
Ok, so you'd like some documentation saying something along the lines of:

> If a buffer with length greater than zero is passed it, it is considered an error of the underlying reader to return `Ok(0)`.

That way you can't necessarily rely on it for memory safety, but you can blame others for bugs :)