Add zk-nullifier example and reorganize zk/ folder #157

Job	Run time
basic-operations/anchor/reinit	7m 59s
ZK zk/zk-id	7m 59s
basic-operations/anchor/update	8m 0s
basic-operations/anchor/burn	8m 0s
basic-operations/anchor/create	7m 59s
ZK zk/zk-merkle-proof	8m 11s
basic-operations/anchor/close	7m 59s
basic-operations/anchor/create-nullifier	8m 0s
ZK zk/zk-nullifier	7m 59s
create-and-update	7m 59s
counter/anchor	7m 59s
	1h 28m 4s

Provide feedback