Inspector: support IDeref

[vemv]

Custom objects implementing IDeref should be navigable to their derefed value.

The resulting UI should be equivalent to the atom one:

i.e. Contains wording, extra level of indentation.

[alexander-yakushev]

The possible problem here could be if the IDeref object is not realized yet. It is arguable whether descending into an unrealized IDeref should trigger its evaluation.

It ie easier if the object also implements IPending and tells us it is realized.

[vemv]

We could also observe a flag passed from cider/cider-nrepl.

Another way to be cautious is to only deref if we're looking at a "leaf" so to speak - if we're inspecting an IDeref, we deref, but if the IDeref is merely an object within the inspected collection, we don't.

[vemv]

I gave this some hammocking and it would seem simple and easy to make down also mean "if the current element is an ideref, deref it".

That way users always see the ideref first, and can explicitly yet easily choose to deref it.