This Coq library provides BigN, BigZ, and BigQ that used to be part of the standard library.
Bignums
The easiest way to install the latest released version of Bignums is via OPAM:
opam repo add coq-released https://coq.inria.fr/opam/released
opam install coq-bignums
To instead build and install manually, do:
git clone https://github.com/coq-community/bignums.git
cd bignums
make # or make -j <number-of-cores-on-your-machine>
make install