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 was actually playing around with doing something like this in a syntax extension, by modifying the AST.  There's some precedent here - [regenerator](http://facebook.github.io/regenerator/) rewrites ES6-style generators into pure ES5 by converting them into a state machine, and I suspect that something like this might be possible with Rust too.  Though, I *also* suspect that it'll be much harder than in Javascript due to potential lifetime conflicts, but that's something I'm not really qualified to talk about :wink: