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)
Ah yes, I think that this should actually return `NonatomicResult<W, Self, W::Err>` so if it fails you get back the whole buffered object again to continue doing whatever you'd like!

We don't currently have an operation for `BufferedReader` with a fixed-size buffer, but I suspect that we can always add one in the future!