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)
More information can always be provided post-1.0. The question here is what `num_cpus` actually means. Since documentation claims to return number of cores available, maybe the name should be changed to `num_cores`?