Weak reference to some object of type G_.
This kind of reference does not prevent the object of type G_ from being reclaimed by the garbage
collector (in which case item returns Void).
Item makes it possible to get (a strong reference to) the object.
Inheriting from this class is prohibited.
Notes: when using the "-no_gc" option (i.e. no garbage collector), the behavior of WEAK_REFERENCE
is equivalent to the behavior of REFERENCE. Also note that WEAK_REFERENCE is not yet implemented
for compile_to_jvm (the behavior is the one of REFERENCE).