Function dataflow::render::reduce::build_distinct

fn build_distinct<G>(
    collection: Collection<G, (Row, Row)>
) -> Arranged<G, TraceAgent<OrdValSpine<Row, Row, <G as ScopeParent>::Timestamp, Diff, usize>>> where
    G: Scope,
    G::Timestamp: Lattice, 

Build the dataflow to compute the set of distinct keys.