Add zk-nullifier example and reorganize zk/ folder #164

Job	Run time
zk/zk-id	9m 50s
basic-operations/anchor/update	6m 23s
basic-operations/anchor/close	9m 50s
basic-operations/anchor/burn	9m 49s
create-and-update	6m 33s
zk/nullifier	9m 50s
basic-operations/anchor/create-nullifier	9m 49s
counter/anchor	6m 31s
basic-operations/anchor/create	9m 51s
basic-operations/anchor/reinit	9m 49s
	1h 28m 15s

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