Add zk-nullifier example and reorganize zk/ folder #172

Job	Run time
zk/nullifier	15s
counter/anchor	14s
basic-operations/anchor/create	15s
create-and-update	14s
basic-operations/anchor/close	14s
basic-operations/anchor/update	14s
basic-operations/anchor/burn	17s
basic-operations/anchor/create-nullifier	14s
zk/zk-id	16s
basic-operations/anchor/reinit	16s
	2m 29s

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