OCaml hash-consing library

The technique is described in this paper:

*Sylvain Conchon and Jean-Christophe Filliâtre.* Type-Safe Modular Hash-Consing.
In ACM SIGPLAN Workshop on ML, Portland, Oregon, September 2006.
The PDF is available at
<https://www.lri.fr/~filliatr/ftp/publis/hash-consing2.pdf>
