input: Write a python function to reverse a string recursively.
  assert rev_string('hello') == 'olleh'
  assert rev_string('world') == 'dlrow'
  assert rev_string('python') == 'nohtyp'
output: def rev_string(s):
      if len(s) == 0:
          return s
      else:
          return rev_string(s[1:]) + s[0]