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 noted in [this comment](https://github.com/rust-lang/rfcs/pull/517#issuecomment-66868363), this is not a declaration that windows does not contain the functionality of unix, but rather the current implementation does not handle the windows/unix split gracefully. Abstractions for windows will be developed independently and if a cross-platform interface can be exposed then it will.