Extrinsic marking of entries with a dedicated file

[lthms]

In addition to marking OCaml types and values with attributes, we can imagine having a way to pass a file as argument, whose content could be used to tweak the generation process of coqffi

For instance

(type
  (name t)
  (coq_model "list"))

would be equivalent to adding [@@coq_model "list"] to the mli file.