input: Write a python function to convert an integer to 64-byte representation.
  assert int_to_int64_bytes(123456789) == b'\x00\x00\x00\x00\x07[\xcd\x15'
  assert int_to_int64_bytes(-987654321) == b'\xff\xff\xff\xff\xc4\xb1\xfb\xef'
  assert int_to_int64_bytes(0) == b'\x00\x00\x00\x00\x00\x00\x00\x00'
output: def int_to_int64_bytes(n):
      return n.to_bytes(8, byteorder='big', signed=True)