RFC: Remove runtime system, and move libgreen into an external library

This RFC proposes to remove the *runtime system* that is currently part of the
standard library, which currently allows the standard library to support both
native and green threading. In particular:

* The `libgreen` crate and associated support will be moved out of tree, into a
  separate Cargo package.

* The `librustrt` (the runtime) crate will be removed entirely.

* The `std::io` implementation will be directly welded to native threads and
  system calls.

* The `std::io` module will remain completely cross-platform, though *separate*
  platform-specific modules may be added at a later time.

[Rendered](https://github.com/aturon/rfcs/blob/remove-runtime/active/0000-remove-runtime.md)

This is a follow-up to [an earlier RFC](https://github.com/rust-lang/rfcs/pull/219).