Fast save_excursion_save and save_excursion_restore.

Fast save_excursion_save and save_excursion_restore.
* lisp.h (struct Lisp_Excursion): New data type.
(PVEC_EXCURSION): New pseudovector type.
(XEXCURSION, XSETEXCURSION, EXCURSIONP): Convenient macros
to deal with it.  Adjust comments.
(init_marker, attach_marker): New prototype.
(unchain_marker): Adjust prototype.
* marker.c (attach_marker): Change to global.
(init_marker): New function.
* alloc.c (Fmake_marker, build_marker): Use it.
(build_marker): More easserts.
(mark_object): Handle struct Lisp_Excursion.
* editfns.c (save_excursion_save, save_excursion_restore):
Reimplement to use struct Lisp_Excursion.  Add comments.