input: Write a python function to sum all the numbers in a list.
  assert list_sum([1, 2, 3, 4, 5]) == 15
  assert list_sum([10, 20, 30, 40]) == 100
  assert list_sum([-1, 1, -2, 2, -3, 3]) == 0
output: def list_sum(numbers):
      return sum(numbers)