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)
By the time the signal is sent, the child can terminate and the PID can be replaced by another. The signal will be sent to the wrong process. It's easy to understand why this is wrong. It's one thing to do this with a command-line utility but it's quite another for processes to be doing this hack on their own accord without a system administrator asking for it.