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: Rust doesn't get to choose how POSIX permissions work. It's not possible for it to override the decision of the platform file system drivers or sysadmin / user configuration. The handling of file systems without POSIX permissions bits seems consistent across platforms.