Changed type of ids_to_inner_sort table used in transformation.

Changed type of ids_to_inner_sort table used in transformation.
It used to be (Cic.id, string) Hashtbl.t, now is (Cic.id, Cic2acic.sort_kind) Hashtbl.t where sort_kind is [ `Type | `Prop | `CProp | `Set ].
String "?", formerly used to denotes meta variable, is not mapped in sort_kind; since it was uniformly treated as "Type", `Type is now used for metas