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)
Can you elaborate on what you think "designed properly" is? The current plan is likely to take the same time argument as the deadline-related functions. The placement in `std::thread` is to emphasize that is only sleeps the current thread.