GC may have inconsistent view of stacktop with deferred references

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In #118450, Sam pointed out that GC can occur at any Py_DECREF. With deferred references, we will need to traverse the object's stacks. The main problem is that frame->stacktop might not represent the true stacktop at a GC collection, because frame->stacktop is not set in between opcodes.

This requires either scanning the entire stack (not just up to stacktop), or deferring GC to safe points.