Update hash_leaves to actually hash and not just concat child nodes

[drewstone]

Our current implementation just adds the two EC points together. We'll want to do a proper hash here, perhaps a pedersen hash eventually.

pub fn hash_leaves(left: MerkleLeaf, right: MerkleLeaf) -> MerkleLeaf {
	return MerkleLeaf::from_ristretto(
		left.0.decompress().unwrap() + right.0.decompress().unwrap(),
	);
}