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 believe we were going to keep the `stdout` object returned will retain the per-object buffering semantics which means that flushes happen when the object is dropped. In that sense I believe that all the buffers will be flushed by process exit as all their destructors should have been called.

Do think it's worth changing to a global `stdout` buffer though? Conceptually it makes more sense to me for `stdin` to be globally buffered than for `stdout`.