Create verifyPerVOSpentVoiceCredits for a certain vote option index

[corydickson]

trafficstars

A public function that has the signature

function verifyPerVOSpentVoiceCredits(
        uint8 _depth,
        uint256 _index,
        uint256 _leaf,
        uint256[][] memory _pathElements,
        uint256 _salt
    ) public view returns (bool)

[weijiekoh]

Context here: https://github.com/ethereum/clrfund/issues/392

[weijiekoh]

_depth can be taken from contract state instead of being passed in as a function parameter

[ctrlc03]

Implement here https://github.com/privacy-scaling-explorations/maci/pull/835