pub fn push(&mut self, leaf: L) {
        let leaf_hash = self.hasher.hash_bytes(leaf.as_ref());
        self.push_hash(leaf_hash);
    }
Adds a new leaf to the tree (replaces leftmost empty leaf).
If the tree is full, its size is doubled.
Note: empty leaves != zero leaves.
core/lib/mini_merkle_tree/src/lib.rs