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)
As I stated above, `fsync` is a valid implementation of `fdatasync`. The `fdatasync` API is nearly exactly the same but it *permits* a more optimized implementation. It's not intended to have a different implementation if there isn't a significant performance win. There is no reason it can't be exposed on Windows. It's exposed on Linux despite it having the same meaning as `fsync` with many file systems... and that is entirely correct.