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)
Also, what if we want to have a `BufferedReader` that has a fixed sized buffer, or one that we construct with a stack `&mut [u8]` slice? It would be a shame for a partial write to force `BufferedReader`s into converting a slice into a `Vec<u8>` in that circumstance.