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)
@thestinger Your attitude makes it very hard for people who agree with you on technical points most of the time (like me) to side with you in discussions. You must realize that the scorched earth approach you seem to be taking isn't likely to achieve your goals; antagonizing people who disagree with you makes it extremely hard (if not impossible) to change their mind.

Please realize that it most definitely isn't enough to just be right; you also need to convince others. Without that, you won't get far.

Again, this is friendly advice coming from someone who has found that they agree with you the vast majority of the time on the technical points and yet still feel like supporting your viewpoints would be the _wrong_ thing to do because of your attitude. _Don't go out of your way to make it hard to agree with you._