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)
Windows has this functionality:

http://msdn.microsoft.com/en-us/library/windows/desktop/aa364944%28v=vs.85%29.aspx

> Symbolic link behavior—If the path points to a symbolic link, the function returns attributes for the symbolic link.