Convert object_tracing to use weak referencesHEADmaster

Object tracing listens to the NEWOBJ hook to see all objects allocated
while it is active. Previously it also enabled a FREEOBJ tracepoint to
drop each object's record as the object was freed.

However the FREEOBJ tracepoint only fires while tracing is active. An
object recorded during tracing can be freed after tracing stops, and
that free was missed, leaving a stale record in object_table keyed by a
freed object. These dangling keys are unsafe during compaction, which
was previously mitigated with rb_gc_pointer_to_heap_p (which I'd like to
stop using). These dangling keys could also become incorrectly
associated with a new object allocated in its place.

Instead we can declare object_table's keys as weak references. The GC
then invokes our weak reference callback on every collection, whether or
not tracing is running, letting us reap the records of objects about to
be freed. This callback runs before the FREEOBJ hook, which makes the
FREEOBJ tracepoint unnecessary.

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