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)
Either name is fine, I was just referring to `StrVector::connect`. I didn’t know about Windows quoting there. I’d say it’s a feature more than a problem, since on Unix `:` is valid in directory names but then that name apparently can’t be used at all in `:`-separated path lists!