Fix and document comparisons in OCaml bindings

[antoinemine]

• remove polymorphic comparison functions for (most) Apron types (all except variables)
• make it clearer in the documentation that these are partially ordered data-structures
• suggest alternate functions in the documentation when equality or partial order testing is needed (as polymorphic =, <=, etc. no longer work)
• rename compare functions as cmp when they actually implement a non-total order, to avoid further confusion and the temptation to use them as keys in sets and maps

These changes aim to address #99.

They may break existing code.

[antoinemine]

Hello. I submitted a PR for Apron 0.9.15 to opam. CI shows that Goblint 2.2.1 and 2.3.0 no longer compile as they reference the compare function we renamed into cmp in this PR, sorry about this. Is that a problem for you or do you see a way around it?