When working with a queryset involving select_related() or

prefetch_related(), it's often useful to be able to check for and

access data returned from the corresponding caches, without the risk of

triggering an unwanted lookup. We use this to conditionally access state

or perform a new query, depending on its presence.
To simplify this pattern, this change introduces

get_object_cached_field(), which returns cached state for a field or

relation on an object.
It first looks up in the field cache (which is populated by

select_related() and may also be populated by a field's implementation

manually).
If not found there, it then checks the prefetch_related() cache.
If still not found, it returns UNSET.

Unit tests pass.
Made use of this in in-progress work.