RFC: IO simplification

This RFC proposes a significant simplification to the I/O stack distributed with
Rust. It proposes to move green threading into an external Cargo package, and
instead weld `std::io` directly to the native threading model.

The `std::io` module will remain completely cross-platform.

[Rendered](https://github.com/aturon/rfcs/blob/io-simplification/active/0000-io-simplification.md)
> I don't want to change anyone's mind

> Everything here has already been approved and the RFC process is a formality here anyway

This seems a bit like a self-fulfilling prophecy. Like @Valloric, I find myself agreeing with your technical points quite often. I have even spent a lot of my personal time arguing for improvements to Rust's IO that look a lot like what you want.

With enough time and effort, I've had reasonable success persuading people of these things (as a non-Mozillian and non core-team member). This RFC is (perhaps ironically) a step in the direction of our shared goals! Sadly, your tactics are poisoning the technical argument, and making it harder for me to argue for them effectively.

If you're burned out on arguing constructively, just let those of us who share your goals make the case. I think you'll be happy with the outcome.