Make "parentless" structs inherit from their builtin type

Make "parentless" structs inherit from their builtin type

* lisp/emacs-lisp/cl-preloaded.el (cl--struct-register-child):
Register child only in struct parents.
(cl-struct-define): Put the "type" as parent of parentless :type structs.
Copy slots only from struct parent classes.
(cl-structure-object): Set (manually) its parent to `record`
and remove assertion that it has no parents.

(cherry picked from commit 7c127fc965fbe781141a6bccbe0b620dc7862b1d)