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)
@vadimcn: How do you expect an application that runs on Windows to make use of `chmod` on \*nix? The only way to do it is by stubbing it out with the same implementation that \*nix uses for FAT32 and NTFS (assuming NTFS isn't mounted in the mode with POSIX permission bits enabled).