input: Write a python function to generate a random string of a given length using available characters.
  assert len(random_string(8)) == 8
  assert len(random_string(16)) == 16
  assert len(random_string(0)) == 0
output: import random
  import string
  
  def random_string(length):
      characters = string.ascii_letters + string.digits + string.punctuation
      return ''.join(random.choice(characters) for _ in range(length))