fix: impl suggestions

[ananas-block]

1. HashSet::find_element_index function, the probe index iteration can be limited to 0..20 similar to insertion, avoiding iteration up to capacity. Use a constant instead of the hardcoded value 20.
2. In HashSet::find_element_iter function, it would be better to iterate from start_iter to (start_iter + num_iterations) as per the parameter name.
3. In IndexedMerkleTree::add_highest_element function, simplify indexed_array.append(&init_value) with indexed_array.init().
4. In IndexedArray::hash_element, simplify hashing by using element.hash(next_element.value).
5. In ConcurrentMerkleTree::append_batch, use self.next_index instead of first_leaf_index + leaf_i.