_gst_swap_objects protects the two oop args, otherwise one of them
can be GC'ed if they're tenured and not accessible from a Smalltalk
context. Why protected them there and not in the code calling them
(grow_dictionary, grow_identity_dictionary, ...) I think it's the
responsibility of the function to protects the objects maybe the
code could change or whatever, it's simpler to follow that rule.