Exercise 3.13: Implemented foldRight2 in terms of foldLeft with tests.
Exercise 3.14: Implemented append in terms of foldRight.
Exercise 3.15: Added stub of concat function with test.
Exercise 3.16: Implemented add1 with test.
Exercise 3.17: Write a function that turns each value in a List[Double] into a String.
Exercise 3.18: Implemented map in terms of foldRight.
Exercise 3.19: Implemented the filter function with tests.
Exercise 3.20: Implemented flatMap in terms of foldRight.