Support `stateMutability` in place of `constant` and `payable`
Merge pull request #62 from f-o-a-m/solc-0.6-support support solc 0.6+ stateMutability instead of constant/payable
Merge pull request #62 from f-o-a-m/solc-0.6-support support solc 0.6+ stateMutability instead of constant/payable