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)
Yes, most of the functionality is indeed already portable! This means that it will be quite easy to re-promote out of `os::platform` :)

> Claiming that Windows doesn't have primary owner/groups, that *nix platforms don't have ACLs or that Windows doesn't have anonymous pipes indicates that the thinking in this RFC isn't ready for prime time.

As I have been trying to clarify in all of my comments, we're not trying to make claims of these forms at all. This RFC is stating that the current implementation does not implement any forms of this functionality, and hence needs to be separated as such.