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)
Note that the usage of "attr" here is only intended for Rust, the functions are not bound to follow OS-specific names. The documentation should definitely be clear, however, about what each platform is corresponding roughly to (e.g. stat() on unix)