New type Lisp_Misc_Ptr

New type Lisp_Misc_Ptr

This is a streamlined version of Lisp_Save_Value, which contains just
a pointer, as that is all Lisp_Save_Values are used for any more.
With the previous changes, these objects are not primarily used as
save values, so just call them "Misc" rather than "Save".
* src/alloc.c (make_misc_ptr): New function.
(mark_object): Mark Lisp_Misc_Ptr too.
* src/lisp.h (Lisp_Misc_Ptr): New constant.
(struct Lisp_Misc_Ptr): New type.
(make_mint_ptr, mint_ptrp, xmint_pointer):
Use Lisp_Misc_Ptr, not Lisp_Save_Value.
(union Lisp_Misc): Add Lisp_Misc_Ptr.
* src/print.c (print_object): Print Lisp_Misc_Ptr.