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)
I think the semantics could be clarified a bit further. For example, I have some code implementing blocking IO on top of non-blocking IO at https://github.com/Ericson2314/rust-net/blob/master/transport/tcp/src/capability/c.rs (aimed at the old semantics). Should I block until non-zero bytes are read/written, but not until the buffer is completely read/written? Seems a bit arbitrary.