feat: add leaf count to `SimpleSmt` (#302)

src/merkle/smt/simple/mod.rs

@@ -152,6 +152,11 @@ impl SimpleSmt {
         <Self as SparseMerkleTree<DEPTH>>::open(self, key)
     }
 
+    /// Returns a count of non-empty leaves.
+    pub fn leaf_count(&self) -> usize {
+        self.leaves.len()
+    }
+
     // ITERATORS
     // --------------------------------------------------------------------------------------------