* Renamed "desc"ription unit to "object" wherever possible.
* Added standard x-ref types to a StandardDomain which is always consulted.
* Split domains module into a subpackage.
* Removed additional_xref_types in favor of new directive classes in StandardDomain.
* Implemented x-ref inventory version 2, for all object types.
* Added env.doc_read_data which is for temporary data stored while reading.
* Minimally updated extension tutorial.
* Started to implement changes to interactive search.
* Test suite passes again.
