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 Java community spent a very long time until they support a platform-neutral API to rename a file (it wasn't supported until Java 7):

- http://stackoverflow.com/questions/13826045/file-renameto-fails
- http://docs.oracle.com/javase/7/docs/api/java/nio/file/Files.html#move%28java.nio.file.Path,%20java.nio.file.Path,%20java.nio.file.CopyOption...%29

They decided to add an extra parameter to control its behavior.  It's not as simple as `.rename(new_path)`.