Changes slug setter to put slugs on a solr document field named 'slug', ...

...instead of 'id', and leaves the 'id' field alone.

Removes the addition of a 'slug' datastream on Fedora objects. It's not being used.
agreed. It is surprising. But I think if you `add` a document that has an ID that already exists, it effectively updates the index, without error, or adding an additional record.

truthfully, idk what the difference between `add` and `update` is w.r.t. Rsolr. Looking that up right now, stand by...