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)
Yeah, I'm well aware that the decisions are all made in advance behind closed doors. All that matters in Rust's development process is who designed it, not the merit behind it. All the evidence from the existing implementation points to it being a dead end, and there's a desperate need for modern AIO support with or without this M:N threading fantasy.