Custom integer encoding size

[epatrizio]

Use a specific OCaml library to specify a custom integer encoding size

For example, with a "classic" encoding (64-bits computer) it's impossible to create the BDD of 8 variables (= truth table of 256 elements) with this integer : 101247173429701308769807673754056716620117458044746632656397076520187575777199

[epatrizio]

Seems that https://github.com/ocaml/Zarith is a good component for that !