native::run is useless

[huonw]

The current implementation is

/// Executes a procedure on the current thread in a Rust task context.
///
/// This function has all of the same details as `start` except for a different
/// number of arguments.
pub fn run(main: proc()) -> int {
    main();
    os::get_exit_status()
}

which is very different to start. It seems it should either be removed, or be adjusted to actually run main inside a Rust context.

(I guess this may be changing dramatically with IO removal.)

cc @aturon, @alexcrichton

[alexcrichton]

Yes I have a feeling that this function has not stood up well to the test of time. I think that libnative itself is slated for removal shortly after the rtio removal, so this will probably just go away with that.