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)
Details like this are generally left to the implementation to sort out, this RFC is already quite large! If possible I'm sure we'll return an `OsStr`, but these details are fairly minor and can probably be hashed out later.