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("abc", "axycb")==False
  assert is_subsequence("xyz", "aaksjdhbsanduyba")==False
  assert is_subsequence("elephant", "energy")==False
  assert is_subsequence("", "anystring")==True
output: def is_subsequence(s, t):
      it = iter(t)
      return all(char in it for char in s)