Reimplement Lisp_Object as pointer-to-incomplete

Reimplement Lisp_Object as pointer-to-incomplete

This makes Lisp_Object values opaque pointers instead of integers,
which helps avoid the same sort of typos that
CHECK_LISP_OBJECT_TYPE helps to avoid, without having to wrap
pointers inside structures.  This also looks forward to supporting
-fcheck-pointer-bounds.
* etc/DEBUG:
* src/.gdbinit (Lisp_Object_Printer.to_string):
Lisp_Object can be a pointer type now.
* src/alloc.c (macro_XPNTR, XPNTR):
* src/emacs-module.c (value_to_lisp_bits, lisp_to_value_bits):
* src/lisp.h (lisp_h_XLI, lisp_h_XIL):
(lisp_h_XUNTAG) [USE_LSB_TAG]:
(XUNTAG) [!USE_LSB_TAG]:
(Lisp_Object, TAG_PTR, make_lisp_symbol):
Support new Lisp_Object implementation as a pointer to an
incomplete type.  Keep pointers pointers, as much as possible.
* src/alloc.c (macro_XPNTR_OR_SYMBOL_OFFSET, XPNTR_OR_SYMBOL_OFFSET):
Remove.  All uses replaced by plain XPNTR.
* src/emacs-module.c: Work around GCC bug 83162.
* src/lisp.h (LISP_WORDS_ARE_POINTERS, lisp_h_XLP, lisp_h_XPL):
(XLP, XPL) [DEFINE_KEY_OPS_AS_MACROS]:
New macros.
(Lisp_Word, untagged_ptr, Lisp_Word_tag): New types.
(XLP, XPL): New inline functions.
(TAG_PTR): Now expands to an initializer, not an expression.
All uses changed.
(TAG_SYMOFFSET, XLI_BUILTIN_LISPSYM): Remove.  All uses removed.
(LISPSYM_INITIALLY): Redo in terms of the new TAG_PTR.
(NIL_IS_ZERO): Redo without XLI_BUILTIN_LISPSYM.
* src/xwidget.c (webkit_javascript_finished_cb): Use XPL
instead of XIL with a non-EMACS_INT arg.
(Fxwidget_webkit_execute_script): Use XLP instead of XLI
followed by two conversions.