Add pure verification method

[saleel]

The verifyProof() method currently does a little more that just proof verification - it store the nullifier in the contract (to prevent verifyProof being called again with same proof).

But there could be use cases where one would want to simply verify the proof - maybe even offchain, and also verify multiple time. Currently this is possible by calling the Verifier contract directly, but the caller would need to get group merkleRoot (and other data ) by calling Semaphore contract first. Additionally they will need to check if the proof was computed using an old merkleRoot themselves (which is already implemented in verifyProof).

We can add another method that does only verification without doing any state changes (saving the nullifier). In fact, this method could be called verifyProof, and the current verifyProof could be something like validateProof() (like validating a ticket which could be done only once).

[cedoor]

I think this makes a lot of sense 👍🏽 I already created an issue for it here #329. I'll close this one and add a reference there to your description.

Please feel free to open a PR if you want!

[saleel]

Oh right, sorry. I remember we creating it, but couldn't find it when I searched yesterday.