Separate supertrait collection from processing a TraitDef. This allows

us to construct trait-references and do other things without forcing a
full evaluation of the supertraits. One downside of this scheme is that
we must invoke `ensure_super_predicates` before using any construct that
might require knowing about the super-predicates.

src/librustc/util/ppaux.rs

@@ -820,9 +820,8 @@ impl<'tcx> Repr<'tcx> for ty::TraitRef<'tcx> {
 
 impl<'tcx> Repr<'tcx> for ty::TraitDef<'tcx> {
     fn repr(&self, tcx: &ctxt<'tcx>) -> String {
-        format!("TraitDef(generics={}, bounds={}, trait_ref={})",
+        format!("TraitDef(generics={}, trait_ref={})",
                 self.generics.repr(tcx),
-                self.bounds.repr(tcx),
                 self.trait_ref.repr(tcx))
     }
 }