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)
Is it worth worrying about the extra word we're paying for here by putting the `PartialResult` in a `Result`? Does anyone have a sense if this will impact tight reader/writer inner loops? I know there's a chance we will get an optimization to squash enums like this into a single tag, but I'm not sure if and when that would happen.