Add zk-nullifier example and reorganize zk/ folder #142

Job	Run time
create-and-update	12m 56s
counter/anchor	12m 16s
basic-operations/anchor/burn	21m 35s
basic-operations/anchor/create	12m 24s
basic-operations/anchor/reinit	22m 20s
basic-operations/anchor/update	12m 47s
basic-operations/anchor/close	21m 8s
basic-operations/anchor/create-nullifier	20m 33s
	2h 15m 59s

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