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)
That's exactly my point. This is really error-prone and should *not* happen. Instead of relying on the library to do everything right, you *yourself* need to think about it. Eventually *some* library will appear, obviously, whether it will be community-driven or "official" one, but it is better not to *force* it given that we can avoid it.