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)
What does it have to do with Linux? The blocking style of programming is completely alien on Windows, it has high-level AIO APIs. Spawning a thread (green *or* native) only to have it block on an I/O request is always going to be a poor way of doing it.