input: Write a python function to convert a word to Pig Latin.
  assert pig_latin('pig') == 'igpay'
  assert pig_latin('latin') == 'atinlay'
  assert pig_latin('apple') == 'appleway'
output: def pig_latin(word):
      vowels = "aeiou"
      if word[0] in vowels:
          return word + "way"
      else:
          return word[1:] + word[0] + "ay"