bug#60102: Move gv-expander of substring to cl-lib

[Juri Linkov]

> The `gv-expander` of `substring` uses the `cl--set-substring` function
> that's defined only in `cl-lib`, so currently, you can compile
>
>     (setf (substring ...) ...)
>
> without requiring `cl-lib` but at run time it will tend to signal
> a `void-function` error.
> We could autoload `cl--set-substring`, but I think a better choice is to
> move this `gv-expander` to `cl-lib.el`.

I tried your patch by evaluating `(tab-bar-make-keymap-1)`,
but it still fails with:

Debugger entered--Lisp error: (void-function cl--set-substring)
  cl--set-substring(#("*scratch* x" 0 9 ...
  tab-bar-auto-width(((sep-1 menu-item " " ignore) (current-tab menu-item 
#("*scratch* x" 0 9 ...
  tab-bar-make-keymap-1()
  (progn (tab-bar-make-keymap-1))
  elisp--eval-last-sexp(nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  command-execute(eval-last-sexp)

OTOH, autoloading `cl--set-substring` fixes this error.