input: Write a python function to square each number in a list using a lambda function.
  assert square_numbers([1, 2, 3]) == [1, 4, 9]
  assert square_numbers([4, 5, 6]) == [16, 25, 36]
  assert square_numbers([7, 8, 9]) == [49, 64, 81]
output: def square_numbers(lst):
      return list(map(lambda x: x**2, lst))