Added a subtraction method

calculator.djinni

calculator =
interface +c {
    # create() is to create an instance of the calculator
    static create(): calculator;
    # summation, adding two i32.. aka integers together. And return an integer.
    summation(number1: i32, number2: i32):i32;

    # look at me, a subtraction method! :)
    subtraction(number1: i32, number2: i32):i32;
}