util reexports less std

sync/src/blocks.rs

@@ -14,6 +14,7 @@
 // You should have received a copy of the GNU General Public License
 // along with Parity.  If not, see <http://www.gnu.org/licenses/>.
 
+use std::collections::{HashSet, HashMap};
 use std::collections::hash_map::Entry;
 use smallvec::SmallVec;
 use util::*;