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)
The only thing I got in return for voicing my opinions on design was a hostile environment where every single attempt at making a contribution meets harsh resistance. It used to be a friendly environment until I decided to voice a dissenting opinion on the design of I/O / concurrency a year ago and it was taken as a personal attack by the people who wrote the runtime. Since then, my pull requests and opinions have become progressively less welcome.

I can't submit a single pull request under my real name without barriers being thrown up and it rotting for ages, and any attempt at an RFC will just be rejected with no justification or taken out of my hands and implemented without giving me any role in the design process or credit for the work I put into it. I know that contributing to Rust is a waste of time when I need to submit any non-bugfix pull request under a pseudonym if I actually want it to be merged.

I now see my involvement with the project in a negative light in the sense that I've spent countless hours of unpaid labour working on a project where I'm no longer welcome. Since that's how I see things, I've chosen to express my negative feelings in my comments for the past months. If I just wanted to pour even more of my time into the project for the largest possible gains, then I wouldn't be putting forwards my unfiltered opinions in a negative tone. Instead, I've chosen to keep contributing on a much smaller scale and I haven't made any attempt to hide how I feel about the project.