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)
There's more to permissions than rwxrwxrwx on *nix. If it's not going to support stuff like ACLs on *nix, it should just be called `chmod`. It doesn't make sense to use different permission systems on different platforms via the same API when ACLs also exist on *nix.