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)
What are these two? For environment variables like `PATH`, with an OS-specific separator? At first, `join_paths` sounds like `Path::join`. Shouldn’t this module just provide the separator, to be used with general-purpose `split` and `connect` methods?