val extension_descr : 
  current_unit:string ->
  Path.t ->
  Types.extension_constructor ->
  Types.constructor_descriptionval labels_of_type : 
  Path.t ->
  Types.type_declaration ->
  (Ident.t * Types.label_description) listval constructors_of_type : 
  current_unit:string ->
  Path.t ->
  Types.type_declaration ->
  (Ident.t * Types.constructor_description) listval find_constr_by_tag : 
  Types.constructor_tag ->
  Types.constructor_declaration list ->
  Types.constructor_declarationval constructor_existentials : 
  Types.constructor_arguments ->
  Types.type_expr option ->
  Types.type_expr list * Types.type_expr listTakes cd_args and cd_res from a constructor_declaration and returns:
- the types of the constructor's arguments
- the existential variables introduced by the constructor

