Arbitrary-precision arithmetic on big integers and on rationals.

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.