Function cd_merkle_root 

pub fn cd_merkle_root(items: &[Hash], padded_len: usize) -> Hash

Returns the root of the “constant-depth” Merkle tree containing the given pre-hashed items or subtree roots plus as many zero-hashes as necessary to bring the number of hashes up to padded_len.

padded_len is expected to be zero or a power-of-2.