input: Write a function to convert a string to a list of characters.
  assert string_to_list('hello') == ['h', 'e', 'l', 'l', 'o']
  assert string_to_list('world') == ['w', 'o', 'r', 'l', 'd']
  assert string_to_list('python') == ['p', 'y', 't', 'h', 'o', 'n']
output: def string_to_list(s):
      return list(s)