Add an example illustration how to plug the C step function of Lustre node from Lurette
Cf examples/lutin/xlurette/. In order to do that, I rely on the ocaml Dynlink support (.cmxs files). The idea is to generate caml code that is able to call the C code (as before) and to generate from that a .cmxs file that registers the OcamlRM functions. I should modify gen_stubs to automate that process.