FC-1426 :vars Relationship binding fix for analytical queries

[bplatz]

New :vars relationship binding support was added for simple-subject-query types, but not legacy analytical query types. This adds support to legacy query types.

The :vars relationship binding feature allows :vars variables to have multiple values (instead of previously there could be only one). They can be supplied with a :vars map with the value(s) being vectors - or :vars can be a vector of maps, with each being a unique set of bound vars for the query.

Using the fluree/chat demo ledger & data, this query now works. Note the ?s variable is bound to the subject, but ?x is what is selected. Two options are provided, and therefore two person results will be returned.

{
  "select": {"?x": ["*"]},
  "where": [["?s", "comment/person", "?x"]],
  "vars": {"?s": [387028092977155, 387028092977153]}
}

[cap10morgan]

👍🏻