HashIndex entries outlive the container

**Issue** `scc::HashIndex` does not drop its entries when the index itself is dropped. **Minimal Reproducer** ```rust #[test] fn entries_should_be_dropped_on_index_drop() { let scc = scc::HashIndex::<u32, Arc<()>>::new(); let test_arc = Arc::new(()); scc.insert_sync(1, test_arc.clone()).unwrap(); drop(scc); let res = Arc::strong_count(&test_arc); assert_eq!( res, 1, "Arc strong count must be 1" ); } ``` **Expected Behavior** When `HashIndex` is dropped, all entries contained within it should also be dropped.