Add verifyPerVOSpentVoiceCredits method to MACI v1.0

[daodesigner]

verifyPerVOSpentVoiceCredits ( voteOptionTreeDepth, _voteOptionIndex, _spent, _spentProof, _spentSalt)

used to verify claim amount in withdraw:
https://github.com/ethereum/clrfund/blob/develop/contracts/contracts/FundingRound.sol#L309

[pettinarip]

@daodesigner just checking the status of this issue. Let me know if we can close or move it to the monorepo as we are cleaning the issues in this repo.