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)
It's not strictly required for either, and doing it for either results in extra unlocked APIs in order to gain back the performance for applications doing a lot of standard stream I/O like Unix-style command-line tools. I don't know what the best approach is, but it's important that it's on par with C / POSIX in terms of letting you work around it.