Take XPNTR private

Take XPNTR private

* src/alloc.c (PURE_POINTER_P): Remove.
All uses replaced with PURE_P.
(XPNTR_OR_SYMBOL_OFFSET): New function.
(XPNTR): Move here from lisp.h.
Reimplement in terms of XPNTR_OR_SYMBOL_OFFSET.
(mark_maybe_object, valid_lisp_object_p, survives_gc_p):
Remove unnecessary cast.
(purecopy): Use XPNTR_OR_SYMBOL_OFFSET instead of XPNTR,
to avoid an unnecessary runtime test for symbols.
* src/lisp.h (lisp_h_XPNTR, XPNTR): Remove, moving XPNTR to alloc.c.
Only alloc.c needs XPNTR now.