Add zk-nullifier example and reorganize zk/ folder #137

Job	Run time
basic-operations/anchor/reinit	1m 6s
counter/anchor	53s
basic-operations/anchor/burn	54s
basic-operations/anchor/close	1m 6s
basic-operations/anchor/create	53s
basic-operations/anchor/update	1m 6s
basic-operations/anchor/create-nullifier	1m 6s
create-and-update	1m 6s
	8m 10s

Provide feedback