Add zk-nullifier example and reorganize zk/ folder #159

Job	Run time
ZK zk/zk-nullifier	11m 41s
counter/anchor	15m 33s
basic-operations/anchor/update	15m 25s
ZK zk/zk-id	12m 44s
basic-operations/anchor/burn	21m 46s
basic-operations/anchor/reinit	21m 10s
basic-operations/anchor/create-nullifier	20m 43s
basic-operations/anchor/create	21m 21s
basic-operations/anchor/close	21m 2s
create-and-update	15m 46s
	2h 57m 11s

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