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)
> The placement in std::thread is to emphasize that is only sleeps the current thread.

All functions block the current thread until they return. The `sleep` function returns when the time is elapsed. I don't see why that makes `std::thread` a good location for it.