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)
This comment is specifically referring to how `io::pipe` is currently only used by `io::process` and alludes to the proposed alternative. It looks like you commented below, however, so I will continue the discussion there instead of here.