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.