Convert a long integer into an octet string.

def i2osp(self, long_integer, block_size):
        'Convert a long integer into an octet string.'
        hex_string = '%X' % long_integer
        if len(hex_string) > 2 * block_size:
            raise ValueError('integer %i too large to encode in %i octets' % (long_integer, block_size))
        return a2b_hex(hex_string.zfill(2 * block_size))