Let "relation" mappers serialize related entities directly
Previously, the set of related entities was computed within "jsonschema.relation" mappers and their serialize() method accepted only the main "entity" from which this "related" set was computed. This is asymmetric with "jsonschema.collection" which serialize() method handles a list of entities. So we make the former mappers accept the set of related directly as an argument to their serialize() method. This set is computed by "jsonschema.entity" mapper. This is a step forward merging "jsonschema.relation" mappers (for non-final relation) with "jsonschema.collection" mappers.