input: Write a python function to convert a given word to Pig Latin form.
  assert pig_latin('apple') == 'appleay'
  assert pig_latin('banana') == 'ananabay'
  assert pig_latin('chair') == 'haircay'
output: def pig_latin(word):
      if word[0] in 'aeiou':
          return word + 'ay'
      else:
          return word[1:] + word[0] + 'ay'