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)
D language allows this formats of `main` function:

```
void main() { ... }
void main(string[] args) { ... }
int main() { ... }
int main(string[] args) { ... }
```

I would really like to see the enforcement placed that `main` must explicitly return the value (I really dislike C's special "this is the function where not-returning value is defined behavior'" just because so many people historically forgot to put `return 0;` at the end.