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)
This doesn't make sense, as explained above. Using `chmod` on a filesystem like FAT32 on \*nix is a no-op just as it is on Windows. Permissions are a property of filesystems, **not** operating systems. There is nothing non-portable about the POSIX permission bits.