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)
If anything dealing with concurrency has to worry about green threads, there's still a big problem. There is no stated use case for this feature and no benefits have been demonstrated. Every benchmark has shown that green threads are entirely useless - why is it sensible to go out of the way to support them?