* lisp/emacs-lisp/checkdoc.el: cl-defstruct + minor simplifications

* lisp/emacs-lisp/checkdoc.el: cl-defstruct + minor simplifications

(checkdoc-make-overlay, checkdoc-overlay-put, checkdoc-delete-overlay)
(checkdoc-overlay-start, checkdoc-overlay-end, checkdoc-char=)
(checkdoc-mode-line-update): Remove old compatibility aliases.
(checkdoc, checkdoc-interactive-loop):
Consolidate common code in if branches.
(checkdoc-error): New struct type.
(checkdoc-error-text, checkdoc-error-start, checkdoc-error-end)
(checkdoc-error-unfixable): Now defined by cl-defstruct.