Add zk-nullifier example and reorganize zk/ folder #166

Job	Run time
counter/anchor	5m 13s
zk/nullifier	10m 17s
basic-operations/anchor/close	11m 14s
zk/zk-id	10m 8s
basic-operations/anchor/update	5m 3s
create-and-update	5m 13s
basic-operations/anchor/create-nullifier	10m 44s
basic-operations/anchor/reinit	10m 58s
basic-operations/anchor/create	11m 7s
basic-operations/anchor/burn	10m 50s
	1h 30m 47s

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