pub fn zero_hash<D: Digest<OutputSize = U32>>(depth: usize) -> [u8; 32]
Cached zero-hash table: zero_hash(d) = hash(zero_hash(d-1), zero_hash(d-1)), with zero_hash(0) == [0u8; 32].
zero_hash(d) = hash(zero_hash(d-1), zero_hash(d-1))
zero_hash(0) == [0u8; 32]