http://benji3up2kxewkqfcq7buxk2xd6zwy3zggnurkrm3l4cvwy2iipvyyad.onion/mirrors/gmpdoc/I_002fO-of-Integers.html
The
integer is written in a portable format, with 4 bytes of size information, and
that many bytes of limbs. Both the size and the limbs are written in
decreasing significance order (i.e., in big-endian).