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)
Like I asked a few comments ago, wouldn’t `exit_with(code)` be a better choice than either setting or returning the code (C/C++ runtime artefact)? This is both safe since it should not modify global state and corresponds to the native APIs better.