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)
It's more convenient to use a high-level AIO API like the one in C# than it is to manage a whole bunch of tasks and deal with message passing. It's also far more efficient and isn't prone to race conditions.