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)
Spawning a task per I/O operation and dealing with message passing is far more verbose and far less inefficient than using a modern language with resumable functions. It's funny that a year after I first brought up these issues, the response is still digging deeper and deeper into this hole with no sign of it getting any better. Perhaps after the 8th or 9th rewrite of the entire I/O library, the message will start to get through.