input: Write a function to determine if the first string is a subsequence of the second. If it is return True else return False.
  assert is_subsequence("abc", "qaxbcy")==True
  assert is_subsequence("axc", "qaxbcy")==False
  assert is_subsequence("abc", "acxb")==False
  assert is_subsequence("abc", "")==False
  assert is_subsequence("", "abc")==True
output: def is_subsequence(s1, s2):
      it = iter(s2)
      return all(char in it for char in s1)