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)
I don't see how it would make *any* sense to set the DOS file attributes or Windows ACLs via the same type that's used on POSIX platforms for the bitflag-based permissions. These are not the same concepts. There are \*nix ACLs, filesystem attribute and extended attributes / MAC labels too. It shouldn't be jammed into one type.