Miscellaneous minor cleanups and simplifications.

Miscellaneous minor cleanups and simplifications.
* lisp/help-fns.el (describe-variable): Don't croak when doc is not found.
* lisp/vc/pcvs.el (cvs-retrieve-revision): Avoid toggle-read-only.
* lisp/menu-bar.el (menu-bar-line-wrapping-menu): Purecopy a tiny bit more.
* lisp/emacs-lisp/syntax.el (syntax-ppss): Simplify with new `if' place.
* lisp/emacs-lisp/smie.el (smie-next-sexp): CSE.
* lisp/emacs-lisp/macroexp.el (macroexp-let2): Fix edebug spec and avoid
((lambda ..) ..).
* lisp/emacs-lisp/eieio.el (eieio-oref, slot-value): Use simpler defsetf.