Make it possible that the mock can call other contracts

[rmeissner]

E.g. add something similar to

function exec(address payable to, uint256 value, bytes calldata data) external {
    bool success;
    bytes memory response;
    (success,response) = to.call{value: value}(data);
    if(!success) {
        assembly {
            revert(add(response, 0x20), mload(response))
        }
    }
}