What's the difference between `arg_types` and `type_params` in `FunctionType`?

[rebcabin]

;;     | FunctionType(ttype*  arg_types,       ;; rename param-types
;;                    ttype?  return_var_type,
;;                    abi     abi,
;;                    deftype deftype,
;;                    string? bindc_name,
;;                    bool    elemental,
;;                    bool    pure,
;;                    bool    module,
;;                    bool    inline,
;;                    bool    static,
;;                    ttype*  type_params,
;;                    symbol* restrictions,
;;                    bool    is_restriction)

[certik]

arg_types is things like integer, real, etc. type_params are template arguments (generics).

[rebcabin]

Reopening as reminder to me to implement