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)
@thestinger I am coming a bit late to this conversation. From a technical perspective, I think [aturon summarized my point of view][at]. However, I do want to draw attention to the language that you used here. Comments like calling the standard library an "incomprehensible puzzle" or a "complete joke" make it difficult to have a productive conversation. Not only are they vague and unspecific, they are just plain rude.

I appreciate that this is an issue you've been talking about for a long time and you feel like we are not listening. In fact, though, your prior comments have been quite influential both on the RFC and the way I personally look at this issue -- I think the same is true for a lot of people. That makes it extra frustrating to read barbed comments like these, which interfere with our ability to have a good, technical back-and-forth discussion that reaches the best overall design.

[at]: https://github.com/rust-lang/rfcs/pull/219#issuecomment-54239182