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)
The fact that it has no *distinct* meaning on Windows isn't a reason to move it to an OS specific module. It can be correctly implemented as a call to `fsync`. It's just an optimization *permitting* but not *requiring* different behaviour. This would just be creating an artificial portability problem.