add exercise difference of squares

Again, no objects, just three separate functions.  This keeps the exercise very simple.  For the difference, solvers can compose the other two functions or write something new.  (Loops can be combined at least, but If they are coding closed formulas, the difference nicely reduces algebraically.)  The three functions are benchmarked separately so solvers can compare the time of the difference function to the sum of times of the other two.