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)
No, they'll stop when all of the other ends of the pipe are closed. It's a leak - you don't know what the lifespan of those file descriptors is going to look like. They can all be sent to a single process over sockets.