Rename Hash::zero() to Hash::default() (#858)

It is more idiomatic, also `zero` is misleading in context of `+` operator.

pool/src/blockchain.rs

@@ -107,7 +107,7 @@ impl DummyChainImpl {
 impl BlockChain for DummyChainImpl {
 	fn is_unspent(&self, output_ref: &OutputIdentifier) -> Result<hash::Hash, PoolError> {
 		match self.output.read().unwrap().get_output(&output_ref.commit) {
-			Some(_) => Ok(hash::Hash::zero()),
+			Some(_) => Ok(hash::Hash::default()),
 			None => Err(PoolError::GenericPoolError),
 		}
 	}

pool/src/pool.rs

@@ -1654,9 +1654,9 @@ mod tests {
 		let mut tx_elements = Vec::new();
 
 		let merkle_proof = MerkleProof {
-			node: Hash::zero(),
-			root: Hash::zero(),
-			peaks: vec![Hash::zero()],
+			node: Hash::default(),
+			root: Hash::default(),
+			peaks: vec![Hash::default()],
 			..MerkleProof::default()
 		};