Add a leaf to the tree.
        :param value: hash value (as a Buffer) or hex string
        :param do_hash: whether to hash value

def add_leaf(self, value, do_hash=False):
        """
        Add a leaf to the tree.
        :param value: hash value (as a Buffer) or hex string
        :param do_hash: whether to hash value
        """
        self.tree['is_ready'] = False
        self._add_leaf(value, do_hash)