Nests with single constructor are ADTs

Before this commit, nests in mutually recursive ADTs with a single
constructor are always considered as enums.

Now, we consider such nests as ADTs.

src/lib/frontend/d_cnf.ml

@@ -747,7 +747,7 @@ let mk_mr_ty_decls (tdl: DE.ty_cst list) =
                 Array.length dstrs = 0 && is_enum
             ) cases ([], true)
           in
-          if is_enum
+          if is_enum && not contains_adts
           then (
             let ty = Ty.tsum name cns in
             Cache.store_ty (DE.Ty.Const.hash ty_c) ty;

src/lib/frontend/typechecker.ml

@@ -2538,6 +2538,11 @@ let rec type_decl (acc, env) d assertion_stack =
   | TypeDecl l ->
     let not_rec, are_rec = partition_non_rec l in
 
+    let not_rec, are_rec =
+      if Lists.is_empty are_rec then not_rec, are_rec
+      else [], List.rev_append not_rec are_rec
+    in
+
     (* A. Typing types that are not recursive *)
     let acc, env =
       List.fold_left