tab-bar.el: Fix "void-function cl--set-substring" error

tab-bar.el: Fix "void-function cl--set-substring" error

* lisp/tab-bar.el: Use #' to quote function names.
Try and fit within 80 columns.
(tab-bar--load-buttons): Silence compiler warnings about `icons` functions.
(tab-bar-auto-width): Avoid the use of `substring` as a gv-place
because it requires `cl-lib` which we don't (want to) preload.
[ Maybe a better solution would be to not preload tab-bar.el.  ]