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)
If you don't know whether it can ever be successful, it belongs nowhere near the standard library. The complexity is unacceptable unless there's a compelling use case for it, and so far green threads have proven to be useless. The implementation exists and it doesn't accomplish anything despite lots of effort being put into it.