Add zk-nullifier example and reorganize zk/ folder #155

Job	Run time
ZK zk/zk-nullifier	13m 4s
ZK zk/zk-merkle-proof	12m 42s
ZK zk/zk-id	12m 36s
basic-operations/anchor/create	21m 7s
create-and-update	15m 44s
basic-operations/anchor/create-nullifier	21m 47s
basic-operations/anchor/reinit	21m 25s
counter/anchor	15m 34s
basic-operations/anchor/burn	21m 42s
basic-operations/anchor/update	15m 34s
basic-operations/anchor/close	21m 20s
	3h 12m 35s

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