Restore Vec::from_iter() specialization

Since I said "no intentional functional change" in the previous commit,
I guess it was inevitable there were unintentional changes. Not
functional, but optimization-wise. This restores the extend
specialization's use in Vec::from_iter.

src/libcollections/vec.rs

@@ -1517,7 +1517,7 @@ impl<T> FromIterator<T> for Vec<T> {
                 vector
             }
         };
-        vector.extend_desugared(iterator);
+        vector.extend(iterator);
         vector
     }
 }