* src/lisp.h (struct Lisp_Fwd): With the predicate enum, we can now pack
the offset and the predicate into a one-word struct.
(XBUFFER_OFFSET): Use the new field name.
* src/buffer.c (DEFVAR_PER_BUFFER): Create the one-word struct.
* src/data.c (store_symval_forwarding): Use the new field name.

Using an enum instead of a symbol makes it obvious that this field is
of no concern to the GC.

* src/lisp.h (enum Lisp_Fwd_Predicate): New.
(struct Lisp_Fwd): Use it instead of a symbol.
* src/buffer.c (DEFVAR_PER_BUFFER): Create the necessary enum constant
instead of a symbol.
* src/data.c (check_fwd_predicate, check_choice): New helpers.
(store_symval_forwarding): Use it.

* src/lisp.h (struct Lisp_Buffer_Objfwd): Deleted.
(struct Lisp_Fwd): Add the fields bufoffset and bufpredicate.
Make the type a 1-byte bitfield so that the entire struct still
fits in two words.
(XBUFFER_OFFSET): Renamed from XBUFFER_OBJFWD.
* src/buffer.c (DEFVAR_PER_BUFFER, defvar_per_buffer)
(buffer_local_value): Update accordingly.
* src/data.c (do_symval_forwarding, store_symval_forwarding)
(set_internal, default_value, set_default_internal)
(Fmake_local_variable, Fkill_local_variable, Flocal_variable_): Use
XBUFFER_OFFSET.

* src/lisp.h (struct Lisp_Kboard_Objfwd): Deleted ...
(struct Lisp_Fwd): ... replaced with field kbdoffset.
(DEFVAR_KBOARD): Use new field.
* src/data.c (XKBOARD_OFFSET): Renamed from XKBOARD_OBJFWD.
(do_symval_forwarding, store_symval_forwarding
(set_default_internal): Use it .

* src/lisp.h (struct Lisp_Objfwd): Deleted.
(struct Lisp_Fwd): Replace it with objvar field.
(DEFVAR_LISP, DEFVAR_LISP_NOPRO, DEFVAR_LISP_NOPROX): Use the field.
* src/lread.c (defvar_lisp): Updated as needed.
* src/pdumper.c (dump_field_fwd): Use the field.
* src/data.c (XOBJVAR): Renamed and updated from XOBJFWD.
(do_symval_forwarding, store_symval_forwarding): Use it.

* src/lisp.h (struct Lisp Boolfwd): Deleted
(struct Lisp_Fwd): Replaced it with a boolvar field.
(DEFVAR_BOOL): Update.
* src/data.c (XBOOLVAR): Renamed from XBOOLFWD.
(do_symval_forwarding, store_symval_forwarding): Use it.
* src/pdumper.c (dump_field_fwd): Use boolvar field.

It was a struct with a single field.

* src/lisp.h (struct Lisp_Intfwd): Deleted.
(struct Lisp_Fwd): Add an intvar field instead.
(DEFVAR_INT): Update accordingly.
* src/data.c (XINTVAR): Updated and renamed from XFIXNUMFWD.
(do_symval_forwarding, store_symval_forwarding): Use it.