Add zk-nullifier example and reorganize zk/ folder #175

Job	Run time
basic-operations/anchor/burn	10m 44s
basic-operations/anchor/create-nullifier	10m 49s
basic-operations/anchor/update	5m 19s
counter/anchor	4m 59s
basic-operations/anchor/create	11m 21s
basic-operations/anchor/close	10m 41s
basic-operations/anchor/reinit	10m 48s
zk/zk-id	10m 58s
create-and-update	5m 12s
zk/nullifier	10m 21s
	1h 31m 12s

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