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)
> Non-blocking I/O is required for correctness in many cases. It's not just about performance, and Rust will not be usable for use cases like process supervision if it doesn't provide it.

To highlight the start of this RFC:

> It is explicitly not a goal at this time to support asynchronous programming models or nonblocking IO, nor is it a goal for the blocking APIs to eventually be used in a nonblocking "mode" or style.

We agree that non-blocking I/O is quite important! We do not, however, have the design bandwidth at this time to stabilize the APIs for 1.0. It's a tough decision to punt on it, but we believe that a solid 1.0 release is more important than stabilizing non-blocking functionality first.