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)
Coming from a sysadmin world, I'd be much happier we actually had `fn main() -> i8 { ... }`. I've seen far too many programs quit printing something like "Error: ...", and leave the exit code as `0`. Is there a reason beyond user experience that we don't want `main` returning an exit code?