Handle inverse property relationships in the rulebase

[nevali]

For example, we might express membership of a dataset as dct:isPartOf, while the dataset itself might express that relationship as <parent> void:subset <child>. In order to process this properly, we need:

• A mechanism for indicating that a particular property expression rule applies in inverse (e.g., void:subset spindle:inverseProperty [ ... ])
• The presence of an inverse property relationship will cause triples containing that predicate to be cached, as it would in a forward relationship.
• When generating caches, ensure triples that satisfy ?other ?p ?source (where ?source is one of the subject URIs for the entity being cached) are available as part of the in-memory source data
• Perform a set of inverse-property queries against the in-memory source data as part of the property matching process (i.e., after forward-matches have been identified, but before losing candidates are discarded)
• When #53 is implemented, encountering ?s ?inverseProperty ?o should mark ?o as needing an update.

[nevali]

Reopening with TODO: cache->refs is not populated when not using the DB index (spindle_cache_source_() needs to be updated, most sensibly within spindle_cache_source_sameas_(), as this already performs the right looping in the right place).