Review API return values

[lread]

I had noticed that the docstringed copy-tree return value did not match reality and snuck in a correction as part of #195.

But... a review suggested that maybe this is not what we want to return for copy-tree.

So... It is probably best to do a quick review of API return values.

Mostly, it will be just the return value from the underlying Java API.

But, for bb fs API fns that do more work, like copy-tree, we probably should be more deliberate in what they return (it currently just happens to return whatever walk-file-tree returns).

We can also decide that a return value is not part of the bb fs API contract. If we do this, it is probably best to be explicit about it in the docstring (and/or README).

I am happy to do the review and whatever PR results from the review.

[borkdude]

Sounds good