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)
Again, `fsync` is an implementation of `fdatasync`. There's nothing lossy / improper about calling `fsync` if the platform doesn't implement this optimization. It's strictly *more portable* than `fsync` because it requires a subset of the guarantees.