input: Write a function to determine the longest of two strings.
  assert longest('Hello', 'World') == 'World'
  assert longest('short', 'longerstring') == 'longerstring'
  assert longest('same', 'same') == 'Strings are equal length.'
output: def longest(str1, str2):
      if len(str1) > len(str2):
          return str1
      elif len(str2) > len(str1):
          return str2
      else:
          return 'Strings are equal length.'