Give the disambiguators a uniform interface
Give the disambiguators a uniform interface

Subclasses can override the following

* _exact_match for exact matching

* _get_possible_entries to return an iterable of possible objects to
  choose from

* _predicate_* to filter out entries

Subclasses of IDConverter must also provide a MENTION_REGEX (can be
None), and override _get_from_id.

This also gives unions a chance to return *exact matches only*.

Ikusaba-san/Chiaki-Nanami