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)
Hm good point! If we were to have a globally allocated instance, however, we do have the infrastructure for [running things at exit](https://github.com/rust-lang/rust/blob/master/src/librustrt/lib.rs#L84-L99) so we could probably schedule it there to drop the global ref count which may end up flushing the buffer.

I do think, however, that we should hammer out the per-object or global semantics here though.