change memory to calldata

[molly-ting]

Changing the data location of function parameters from memory to calldata can save much gas. In the following example, test4 saves about 955 units of gas.

    function test3(uint256[] memory amounts) public {
        uint256 amount = amounts[0];
    }

    function test4(uint256[] calldata amounts) public {
        uint256 amount = amounts[0];
    }

[molly-ting]

Sorry, I just found that calldata is typically used in external function and the functions I changed are private or internal. I'll close this pull request.