Replace all assert! with debug_assert!

This results in a ~6% reduction in instruction count.

src/domtree.rs

@@ -33,7 +33,7 @@ fn merge_sets(
             node2 = idom[node2.index()];
         }
     }
-    assert!(node1 == node2);
+    debug_assert!(node1 == node2);
     node1
 }