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)
Not currently, we do not want to bind ourselves to msvcrt. Similar functionality can be exposed through its own interface at the `std::os` level if necessary, but we want to only require kernel32.dll on windows.