Integrate the new `Metadata` object as `meta` property on `Object`.
Use it to actually store metadata after a successful stage run.
A new class `PathAdapter` is introduce which is in turned used to
expose the base path of `Object` as `os.PathLike` so it can be
passed as path to `Metadata`. The advantage is that any changes
to the base path in `Object` will automatically be picked up by
`Metadata`; the prominent, and currently only, case where this is
happening in `Object` is `store_tree`.