pp.el (pp-default-function): New custom var

pp.el (pp-default-function): New custom var

* lisp/emacs-lisp/pp.el (pp-use-max-width): Make obsolete.
(pp-default-function): New custom var.
(pp--object, pp--region): New helper functions.
(pp-29): New function, extracted from `pp-to-string`.
(pp-to-string): Add `pp-function` arg and obey `pp-default-function`.
(pp-28): New function, extracted from `pp-buffer`.
(pp-buffer): Rewrite, to obey `pp-default-function`.
(pp): Obey `pp-default-function`.
(pp-emacs-lisp-code): Add new calling convention to apply it to a region.