input: Write a python function to calculate the area of a square and a rectangle.
  assert AreaCalculator().square_area(4) == 16
  assert AreaCalculator().square_area(5) == 25
  assert AreaCalculator().rectangle_area(4, 5) == 20
  assert AreaCalculator().rectangle_area(6, 7) == 42
output: class AreaCalculator:
      def square_area(self, side):
          return side * side
  
      def rectangle_area(self, length, width):
          return length * width