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)
> We don't know that it's useless.

The only argument that makes sense for inclusion in the stdlib is  "we know that it is useful". We don't *know* that `f(x, y) = x * hamming_weight(y)` is useless, but it would be complete nonsense to add it to the libraries.

(Not expressing an opinion either way, just pointing out that we should not be using this as a criterion for additions.)