Add zk-nullifier example and reorganize zk/ folder (#28) #176

Job	Run time
zk/zk-id	11m 11s
basic-operations/anchor/create	11m 28s
basic-operations/anchor/update	5m 11s
counter/anchor	5m 2s
basic-operations/anchor/create-nullifier	11m 53s
basic-operations/anchor/reinit	11m 23s
create-and-update	5m 14s
basic-operations/anchor/burn	11m 16s
basic-operations/anchor/close	11m 22s
zk/nullifier	10m 36s
	1h 34m 36s

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