explicit returns: copy-tree now returns target-dir

Step 3a for #197 (comment)

`copy-tree` return was formerly undocumented and happened to return
`source-dir`.

For consistency with `copy` and other fns, `copy-tree` now returns
`target-dir` and is now explicitly documented as doing such.

The bb fs API is deliberately vague about return types, but to be
consistent with most other API fns, we return `target-dir` as a `Path`.

Adjusted tests accordingly.

Author: lread