Add zk-nullifier example and reorganize zk/ folder #165

Job	Run time
basic-operations/anchor/create-nullifier	10m 46s
counter/anchor	5m 12s
create-and-update	5m 11s
basic-operations/anchor/create	10m 47s
zk/zk-id	10m 8s
basic-operations/anchor/burn	11m 10s
basic-operations/anchor/close	10m 52s
basic-operations/anchor/reinit	10m 36s
basic-operations/anchor/update	5m 5s
zk/nullifier	10m 35s
	1h 30m 22s

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