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)
Does [this comment](https://github.com/rust-lang/rfcs/pull/517#issuecomment-66868363) help clear it up for you? Movement into `os::platform` is emphasizing that implementations are lacking today, but have the space to grow into official cross-platform abstractions later.