Add zk-nullifier example and reorganize zk/ folder #158

Job	Run time
ZK zk/zk-nullifier	4m 9s
counter/anchor	4m 8s
basic-operations/anchor/create-nullifier	4m 8s
basic-operations/anchor/update	4m 9s
basic-operations/anchor/burn	4m 9s
ZK zk/zk-merkle-proof	4m 9s
ZK zk/zk-id	3m 29s
basic-operations/anchor/close	3m 14s
basic-operations/anchor/create	4m 8s
create-and-update	3m 25s
basic-operations/anchor/reinit	3m 19s
	42m 27s

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