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 does permit the omission of an explicit return statement in `int main(void) {}`, in which case it acts as if a `return 0;` was placed at the bottom of the function. Allowing either `fn main() {}` or `fn main() -> i8 {}` or whatever is a less weird approximation.