Some convenient bits to deal with Lisp_Save_Values.

Some convenient bits to deal with Lisp_Save_Values.
* lisp.h (XSAVE_OBJECT): New macro to extract saved objects.
(allocate_misc): Remove prototype.
(format_save_value): New prototype.
* alloc.c (allocate_misc): Revert back to static.
(format_save_value): New function to build Lisp_Save_Value
object with the specified internal structure.
(make_save_value): Reimplement using format_save_value.
* editfns.c (save_excursion_save): Use format_save_value.
(save_excursion_restore): Use XSAVE_OBJECT.