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)
Can you clarify what `File` means here?  Is it for "regular files" in Unix terminology (ie. does it exclude named pipes, symlinks and directories)?  If so, will there be no ways to rename pipes, symlinks and directories?  Named pipes are specific to Unix but the other two are also supported by Windows.