Add zk-nullifier example and reorganize zk/ folder #160

Job	Run time
ZK zk/zk-nullifier	11m 8s
basic-operations/anchor/burn	21m 20s
basic-operations/anchor/close	20m 30s
counter/anchor	15m 39s
ZK zk/zk-id	12m 4s
basic-operations/anchor/create	21m 11s
basic-operations/anchor/create-nullifier	20m 47s
create-and-update	15m 23s
basic-operations/anchor/update	15m 55s
basic-operations/anchor/reinit	20m 59s
	2h 54m 56s

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