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)
FreeBSD does not support `fdatasync(2)` and `liblibc` has a binding to it only on Linux.  It's odd to move it under the `unix` module.

- https://github.com/freebsd/freebsd/blob/master/include/unistd.h#L386
- https://github.com/rust-lang/rust/blob/master/src/liblibc/lib.rs#L289

FYI: it is a part of POSIX Realtime Extension rather than POSIX in a narrower sense:

- http://pubs.opengroup.org/onlinepubs/9699919799/functions/fdatasync.html