message: Migrate to crossbeam's new 0.4 API
diff: @@ -6,7 +6,8 @@ use std::mem;
  use std::sync::mpsc::{channel, Receiver, Sender};
  use std::sync::Arc;
  -use crossbeam::{self, Scope};
  +use crossbeam;
  +use crossbeam::thread::Scope;
  use jobserver::{Acquired, HelperThread};
  use core::profiles::Profile;
  