Support 64-bits integers (#7)

[adrien-n]

This patch series functorizes the Simple module over integer handling in order to optionally support 64 bits integers. The API doesn't expose the functor but merely two modules: Simple and Simple64. The Simple module retains the same API while the Simple64 one use int64 instead of int where the values might overflow the range of int.

In addition to that, the tesuite is updated with a new test which is not in the "appendix-a.json" testuite, both Simple and Simple64 are tested, and dune is made to provide a diff against a reference output for improved readability.

[c-cube]

seems like a git merge gone wrong?

[adrien-n]

Definitely. I've fixed it, thanks.

[adrien-n]

I've updated my branch with the fix for the merge issues mentionned by @c-cube and a bad assert which checked > 0 instead of >= 0 for integers.

(and thanks @c-cube for the CI changes :) )