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've been quite successful in influencing Rust's development with a diplomatic approach. My almost entirely negative attitude towards the project has everything to do with how I've been treated and nothing to do with being burned out on arguing constructively.