Add zk-nullifier example and reorganize zk/ folder #148

Job	Run time
basic-operations/anchor/close	21m 39s
basic-operations/anchor/create-nullifier	21m 30s
basic-operations/anchor/reinit	21m 12s
counter/anchor	15m 5s
basic-operations/anchor/update	15m 32s
basic-operations/anchor/create	15m 45s
basic-operations/anchor/burn	21m 24s
create-and-update	15m 11s
	2h 27m 18s

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