input: Write a python function to combine each character of two strings into all possible pairs.
  assert combine_strings('a', 'b') == ['ab']
  assert combine_strings('ab', 'cd') == ['ac', 'ad', 'bc', 'bd']
  assert combine_strings('123', '45') == ['14', '15', '24', '25', '34', '35']
output: def combine_strings(s1, s2):
      return [a + b for a in s1 for b in s2]