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)
You may want to take this discussion to the [design principles](https://github.com/aturon/rfcs/blob/io-os-reform/text/0000-io-os-reform.md#design-principles) section of this RFC instead because this decision is just following those principles. The `datasync` operation does not exist on Windows (as far as we know), and exposing this directly contradicts the princple "The APIs should only expose a service or a configuration if it is supported on all platforms"