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)
@erickt: It needs to be clearly defined as either online cores or configured cores. If it's the number of online cores, then you need to deal with the edge case of newly added cores in something like a per-CPU cache.