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)
It's not complex: park/unpark is simple. I disagree with "if you don't know whether it can ever be successful, it belongs nowhere near the standard library": we are closing off the possibility of any Quasar-like green threads *ever* if we don't at least have a park/unpark abstraction. What we are doing is the conservative choice.