Currently trans uses Vec<ty::t> to represent substitutions instead of a proper

ty::substs struct. This is a holdover from the olden days of yore. This patch
removes the last vestiges of that practice. This is part of the work
I was doing on #5527.

src/librustc/util/ppaux.rs

@@ -561,6 +561,12 @@ impl Repr for ty::substs {
     }
 }
 
+impl Repr for ty::ItemSubsts {
+    fn repr(&self, tcx: &ctxt) -> ~str {
+        format!("ItemSubsts({})", self.substs.repr(tcx))
+    }
+}
+
 impl Repr for ty::RegionSubsts {
     fn repr(&self, tcx: &ctxt) -> ~str {
         match *self {