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)
Since we're already renaming this, why not just go with `receive` instead of `recv`? There's no point to the abbreviation. Readability, familiarity, simplicity and plain obviousness are far more important that saving a couple of keystrokes.