Refine Awol model

- awol:Content can simply be equivalent to “anything with an awol:body
  or an awol:src”, and its restrictions are consequently inherited by
  any object which uses those properties. This simplifies the model,
  and makes it better match intended usage. awol:Content and awol:Link
  are made disjoint to prevent confusion with awol:lang and awol:type,
  which can (now) apply to either.

- awol:lang in particular is refined to not just provide the language
  of awol:Link¦s, but potentially any resource (as is true of
  awol:type). However, a domain restriction is made: If awol:Content
  has an awol:body which is a language‐tagged string, it must not also
  have an awol:lang. This continues to offer two possible encodings for
  plaintext content, one as an xsd:string with an awol:type of
  `text/plain` and an awol:lang, and another as a language‐tagged
  string with no awol:type or awol:lang. The latter is preferred.

- Many things are now classified as foaf:Document¦s which previously
  weren¦t, but reasonably should have been.