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)
Also, does this mean we are going to have a global buffer for these like C? It's a performance regression but it does provide nicer semantics. If it's a per-handle buffer with different handles everywhere, then the semantics are very weird.