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)
As mentioned by [this comment](https://github.com/rust-lang/rfcs/pull/517#issuecomment-66868363), this is not a declaration that windows does not have users or groups. It is an indication that these are currently only implemented for unix. Once we have a full cross platform implementation we can move them back onto `Command` directly (if possible).