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)
I would be okay with `.vars()` returning an iterator that held a lock. In the worst case, it's a pretty straightforward deadlock to document and find, and if we ever get eager drops then this wouldn't be an issue anymore.