input: Write a python function to find the longest string in a list.
  assert longest_string(['hello', 'world', 'abracadabra']) == 'abracadabra'
  assert longest_string(['Python', 'is', 'awesome']) == 'awesome'
  assert longest_string(['short', 'longer', 'longest']) == 'longest'
output: def longest_string(strings):
      return max(strings, key=len)