mapping(bytes32 => uint256) internal uintStorage;
mapping(bytes32 => string) internal stringStorage;
mapping(bytes32 => address) internal addressStorage;
mapping(bytes32 => bytes) internal bytesStorage;
mapping(bytes32 => bool) internal boolStorage;
mapping(bytes32 => int256) internal intStorage;
Taken from: https://github.com/zeppelinos/labs/blob/master/upgradeability_using_eternal_storage/contracts/EternalStorage.sol