input: Write a python function to find the longest word in a list of words.
  assert longest_word(["cat", "dog", "elephant", "rhinoceros"]) == "rhinoceros"
  assert longest_word(["apple", "banana", "cherry", "date"]) == "banana"
  assert longest_word(["a", "ab", "abc", "abcd"]) == "abcd"
output: def longest_word(words):
      return max(words, key=len)