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)
@bill-myers: Cancelling a task would leak all of the resources. Also, what happened to *not requiring a runtime*?

@nodakai: How do you plan on cancelling the pending `read` and `write` calls? Is it going to install signal handlers?