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)
There's some discussion of this a couple lines down. `stdin` was recently updated to be a global buffered reader, but the need to do that for the writers is less clear since you don't lose stuff by accident in those cases.