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)
Agreed. `FILE` is just a C API that provides buffered IO, but which we have no control over. Buffering isn't too hard, and having direct control over the heuristics and the API surface makes more sense than tying ourselves to whatever legacy semantics the C API happens to have.