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)
Making it much more difficult to support features like non-blocking I/O in the future that are actually *required* for correct handling of subprocesses. It's impossible to do it correctly with blocking I/O, because you need to read from the child as you write in order to guarantee that the writes don't block forever as the child is blocking on write.