input: Write a python function to double each element in the given list.
  assert double_elements([1, 2, 4, 8, 16]) == [2, 4, 8, 16, 32]
  assert double_elements([3, 6, 9]) == [6, 12, 18]
  assert double_elements([0, -1, -2]) == [0, -2, -4]
output: def double_elements(lst):
      return [x * 2 for x in lst]