Support disjoint unions; improve modelling somewhat

- `anno:Selector` can no longer be refined by `anno:State` (in cases
  where an object is both).

- `awol:FeedOrEntry` is now formalized as a disjoint union of
  `awol:Feed` and `awol:Entry`. This was already true, as Feeds and
  Entries were disjoint and Feed Or Entry was equivalent to their
  union, but this states it explicitly.

- `dcterms:LicenseDocument` is now a subclass of `foaf:Document`.

- Plumbing `dcterms:` classes which simply indicated one of a variety
  of classes are now equivalent to their union, not simply superclasses
  of them. This simplifies the ranges of properties which previously
  indicated the union and aligns with how these classes were probably
  intended. This union is _not_ defined as disjoint.

- Superfluous disjoint rules were removed from `foaf:Agent` and
  `foaf:Document`; these can be inferred.