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)
All evidence we have so far indicates that it's useless and doesn't fit in with the language's niche. On the other hand, there's nothing to back up the design that's proposed here. Where are the performance numbers showing that there's a use case for this?