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)
The Windows permission model has users and groups just like the POSIX permission model. It also has the concept of a primary user and group just like the POSIX permission model. The `chmod` function behaves the same way on \*nix for filesystems like FAT32/NTFS as it does on Windows. Both platforms also have the concepts of ACLs.