Allow resources to belong to more than 1 collection/subdividing a collection

We have some collections which would benefit from the ability of resources to be subdivided further. For example, fonds of an archives.

Playlists are the current method to do this, however, are not that visible to a user. It would be beneficial to either allow resources to live in multiple collections, or to allow collections to be subdivided in some way (e.g. through a filer or facet, accessible from the collection detail page)