Add zk-nullifier example and reorganize zk/ folder #132

Job	Run time
basic-operations/anchor/create-nullifier	42s
basic-operations/anchor/reinit	30s
basic-operations/anchor/update	30s
create-and-update	28s
basic-operations/anchor/create	29s
basic-operations/anchor/close	30s
basic-operations/anchor/burn	29s
counter/anchor	29s
	4m 7s

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