Add zk-nullifier example and reorganize zk/ folder #152

Job	Run time
basic-operations/anchor/create-nullifier	22m 11s
create-and-update	15m 25s
counter/anchor	16m 2s
basic-operations/anchor/close	21m 5s
basic-operations/anchor/burn	21m 48s
basic-operations/anchor/reinit	21m 24s
basic-operations/anchor/update	16m 7s
basic-operations/anchor/create	21m 11s
	2h 35m 13s

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