Add zk-nullifier example and reorganize zk/ folder #173

Job	Run time
create-and-update	5m 4s
zk/zk-id	5m 51s
basic-operations/anchor/create-nullifier	5m 50s
basic-operations/anchor/update	4m 52s
counter/anchor	5m 3s
basic-operations/anchor/reinit	5m 50s
basic-operations/anchor/create	5m 50s
basic-operations/anchor/close	5m 51s
basic-operations/anchor/burn	5m 50s
zk/nullifier	5m 49s
	55m 50s

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