Support asset re-use across scenes

[mjkalyan]

Currently, unload-scene-* functions free both objects and assets associated with a scene. Typically, we'd have our *assets* somewhere globally and our make-scenes will reference that asset list. When we free one scene using something in *assets* we'd need to reload it for another scene to use it. Maybe we need to split %free into %free-assets and %free-objects. Same thing when scene parameters make their way into the mix.

[mjkalyan]

%free :later or %free :never would work, of course, but that would also keep the objects around.

[shelvick]

Sure, but let's keep :free as a convenience feature to hold onto everything.