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)
`file_attr` doesn't sound right to me (there are several things called "file attributes" (e.g. `GetFileAttributes`), and this function returns none of them). I think `stat` is a fine name, Windows calls this `GetFileInformationByHandle`/`NtQueryInformationFile`, and `file_info` could be also a decent name (but it feels way too non-specific to me), but on Windows you typically get this from `FindFirstFile`, which is rather sucky.