This is an attempt to make the lifecycle of wrapped objects clearer.
Godot keeps track of bindings' userdata for each object it creates.
This allows allocating the memory of the wrapper only once per object
even if that object is passed multiple times between binding code and
godot code.
The binding information is composed of multiple functions, this includes
a callback for when the userdata is to be allocated (called once) and
for when the userdata is to be deallocated (again, called once).
When allocating data with "memnew" we set the object bindings during the
postinitialize phase, but surely we shouldn't do that when allocating
the userdata as a result of bindings callback themselves.
Additionally, since we let Godot handle (and track) raw memory
allocation and de-allocation, we need to manually call the deconstructor
of the wrapper class during the free callback, to ensure that its
non-trivial members are correctly de-initialized.
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