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)
This is a massive understatement. It adds a substantial amount of complexity, to the point where it takes at least ten times as much code to support anything as it would otherwise. Just look at how complex it is to support mutexes and thread-local storage - there's at least 500x more code than there would be otherwise, and that's not even hyperbole. It's misleading to phrase this as "somewhat more complex".