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)
I assume they correspond to POSIX C `SEEK_CUR`, `_END`, and `_SET`.  How about adding Linux specific `SEEK_DATA` and _`HOLE` for manipulating sparse files (*BSD also added them)?  Python 3.3 decided to support them: 

- http://bugs.python.org/issue10142

It seems to be possible to support them on Win32, though I haven't checked the details:

- http://msdn.microsoft.com/en-us/library/windows/desktop/aa365566.aspx Sparse File Operations (Windows)