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)
I don't think this makes sense. The `chown` function is also a no-op on Linux for certain file systems. It makes sense to expose a portable `chmod` function because it's *already* best effort on every platform.