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)
@nodakai: UTF-8 usually saves memory for Japanese in plain text formats like XML/HTML/JSON, including ebooks. There has to be very little formatting for UTF-16 to win. It will lose consistently for a corpus like Japanese Wikipedia pages as HTML and it won't be a clear win for the underlying MediaWiki mark-up.