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'm not going to show respect to people who haven't shown me any. I think Rust's standard library and development process are a joke and I'm not going to filter my thoughts on it. Treating volunteer contributors like shit and then expecting respect from them is insane.