ocaml-num is deprecated - switch to Zarith is recommended

[XVilka]

Previously Num was a part of OCaml compiler. Recently it was extracted out of it, and deprecated if favor of Zarith:

This is a legacy library. It used to be part of the core OCaml distribution (in otherlibs/num) but is now distributed separately. New applications that need arbitrary-precision arithmetic should use the Zarith library (https://github.com/ocaml/Zarith) instead of the Num library, and older applications that already use Num are encouraged to switch to Zarith. Zarith delivers much better performance than Num and has a nicer API.

from the https://github.com/ocaml/num/