input: Write a function to convert an integer to an 8-bit binary string.
  assert int_to_bin(10) == '00001010'
  assert int_to_bin(255) == '11111111'
  assert int_to_bin(0) == '00000000'
  assert int_to_bin(1) == '00000001'
output: def int_to_bin(n):
      return format(n, '08b')