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)
Exposing `/tmp` or `TMPDIR` like this is asking for trouble. It's not secure to directly create files there. It should be done via an API like `tmpfile` so I don't see what there is to gain from exposing this.

In many cases the per-user cache directory or runtime directory are what's actually wanted anyway.