input: Write a python function to check if a string is a subsequence of another string.
  assert is_subsequence("abc", "ahbgdc") == True
  assert is_subsequence("axc", "ahbgdc") == False
  assert is_subsequence("hello", "he l l o world") == True
output: def is_subsequence(s, t):
      it = iter(t)
      return all(char in it for char in s)