Make satML types abstract

[bclement-ocp]

          I believe both `Satml_types` and `Satml` modules actually sort of need to know its internals currently, so nothing major as far as I know. Making it an abstract type is not entirely trivial refactoring, but we can try to preemptively not make it too hard on ourselves for when we actually do it :)

Originally posted by @bclement-ocp in https://github.com/OCamlPro/alt-ergo/pull/961#discussion_r1400862681

Note: this is a refactoring I don't think we should do just for the sake of doing it, it is more something to keep in mind if we can include into architectural changes to Satml — this change would probably be quite a bit of churn that "for the sake of abstract types" alone is probably too weak a reason