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)
Even if the rust code doesn't need or use extra synchronization, I am partial to making the stream splitting explicit, so freestanding implementation of the rust standard library can, e.g., lock the incoming and outgoing buffers separately. (Of course, this is a very hypothetical / niche usecase.)