Function zero_hash 

pub fn zero_hash<D: Digest<OutputSize = U32>>(depth: usize) -> [u8; 32]

Compute zero_hash(depth), where zero_hash(d) = hash(zero_hash(d-1), zero_hash(d-1)) and zero_hash(0) == [0u8; 32]. Recomputed per call (not memoized).