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)
A decent way of doing it would be to have a "supervisor task" that catches unhandled exceptions from the main task and prints them to stderr. We do have to be careful not to trigger libc multithreading pessimisations, through (so it should try not to call libc).