Improve/fix JSON deserialization

This merge request moves the inverse dicts from the JSON deserialization to _generic.py, since they are used in the XML deserializaton as well. It also makes _construct_reference() return its object_class parameter, as mentioned on zulip nearly two months ago. :'D

added 2 commits

• 0aa364d8 - adapter: move inverse dicts to _generic
• e1ec36c2 - adapter.json: return object_class in _construct_reference()

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changed title from Fix/move inverse dicts to Improve/fix JSON deserialization

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mentioned in commit 6278d03f