Implementing finalization hooks.

* gc.c (struct fin_reg): New struct type.
(final_list, final_tail, mark_makefresh): New static variables.
(mark_obj): Under generational GC, if make_makefresh is in
effect, set the generation to -1 on all marked objects.
(sweep_one): In an EXTRA_DEBUGGING  build, call breakpt
if the object being swept is the one in break_obj.
Under generational GC, place reachable objects that are
in generation -1 the freshobj nursery and assign them to generation 0,
rather than sticking them into the mature generation 1.
(sweep): Under generational gc, reset the freshobj_idx variable
here, so that sweep_one has an empty nursery in which
to place the generation -1 objects.
(prepare_finals, call_finals): New static functions.
(gc): Call prepare_finals before sweep, and call call_finals
just before re-enabling GC and returning. Do not reset freshobj_idx to
zero; this was done in sweep, which may have added entries into it.
(gc_finalize): New function.
(gc_late_init): Register gc_finalize as intrinsic function finalize.

* txr.1: Documented finalize.

* HACKING: Documented finalization, described the additional
meaning of the -1 generation, and added a section on debugging with
break_obj and breakpt.

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