input: Write a function to convert a comma-separated string to a list of integers.
  assert to_int_list("1,2,3,4,5") == [1, 2, 3, 4, 5]
  assert to_int_list("10,20,30") == [10, 20, 30]
  assert to_int_list("7,8,9") == [7, 8, 9]
output: def to_int_list(s):
      return list(map(int, s.split(',')))