feat: refactor simple smt to use empty subtree constants

Prior to this commit, there was an internal procedure with the merkle
trees to compute empty sub-tree for arbitrary depths.

However, this isn't ideal as this code can be reused in any merkle
implementation that uses RPO as backend.

This commit introduces a structure that will generate these empty
subtrees values.

benches/smt.rs

@@ -18,7 +18,7 @@ fn smt_rpo(c: &mut Criterion) {
                     (i, word)
                 })
                 .collect();
-            let tree = SimpleSmt::new(entries, depth).unwrap();
+            let tree = SimpleSmt::new(depth).unwrap().with_leaves(entries).unwrap();
             trees.push(tree);
         }
     }