input: Write a function to calculate the current fitness of a given population.
  assert calculate_fitness([1, 2, 3, 4, 5]) == [0, -1, 0, 3, 8]
  assert calculate_fitness([-1, -2, -3]) == [8, 15, 24]
  assert calculate_fitness([10, 20, 30]) == [63, 303, 663]
output: def calculate_fitness(population):
      return [x**2 - x for x in population]