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)
@thestinger 

If you are not doing anything weird, the threads will die along with the subprocess when the subprocess terminates and the reader gets an empty read/the writer gets EPIPE, and you still need to send things to the parent over a channel. Of course, starting 2 new threads for a process is rather wasteful and unnecessary when you could just select(2).