Further improvements to narrowing locks

Further improvements to narrowing locks

* src/editfns.c:
(narrowing_lock_get_bound): Return a pointer to a struct
Lisp_Marker instead of a character position.  Suggested by Eli
Zaretskii.
(reset_outermost_narrowings, unwind_reset_outermost_narrowing)
(Fwiden, Fnarrow_to_region): Adapt accordingly.
(narrowing_lock_peek_tag, narrowing_lock_push)
(narrowing_lock_pop, narrowing_locks_save)
(narrowing_locks_restore): Use XCAR/XCDR/XSETCAR instead of
Fcar/Fcdr/Fsetcar.