Special lists must incref the owning datum

[tontyGH]

• atom.contents

• atom.locs (does not apply)

• atom.vis_contents

• atom.vis_locs

• atom.overlays / atom.underlays

• atom.vars

• atom.verbs

• atom.filters (does not apply)

• client.verbs (likely applies but clients have special ref handling that makes this irrelevant, maybe for completeness?)