Deserialisation via FieldInfo.SetValue now preserves nested object graphs

Deserialising nested object graphs with FieldInfo.SetValue now works reliably. Previously, when a deserialisation routine stored a class-typed child object into a parent field and then returned the parent, the child object was destroyed as soon as the routine's local references went out of scope. Reading the child afterwards raised NothingReferenceException ("Cannot call method 'ToString' because the receiver is Nothing"), even though the same read worked inside the routine. Primitive fields (Integer, String, Boolean, Double) were unaffected. Overwriting an object-valued field also leaked the previous value.