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)
Argh! The renaming to `BufferedReader` came late and I didn't catch this.

I'm not particularly happy with the trait name. @alexcrichton prefers `Buffered`, but I worry that we may eventually want something for buffered writers.

Suggestions welcome.

re: `with_capacities`, I agree; we could simplify for now, and add back this functionality later if needed.