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)
The idea was that people would be doing a lot more operations on strings from the Rust side than in WinAPI calls, so that the performance gained by cheap conversions between Wtf8 and utf-8 outweighs the cost of converting to and from UCS-2 for WinAPI calls.
As far as I know, nobody has done any benchmarking to determine whether that is actually true.