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)
It might also be worth thinking about if we want to keep `BufferedStream::with_capacities` as is, or have it take a single size that's used for both buffers. I'm not really sure if anyone wants different buffer sizes for readers and writers.