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)
While it might make sense to rename to a long identifier in this particular case, I'd like to at least be a little careful that we don't create a language that too heavily relies on IDE/editor features like code-completion to write.