Pekko Streams: use arena allocation

[Ostrzyciel]

In #373 we are adding an arena allocator for rows, triples, and quads. Currently, the stream module doesn't use that, but rather plain old immutable structure allocation. This makes sense, as we are letting the user pass around RdfStreamRow and RdfStreamFrame objects – who knows what is happening to them in the application.

For this, combined-stage parsers/deserializers would be needed, hiding away what's happening internally from the user. This would also be needed to implement #371

[Ostrzyciel]

For parsing, this should use the solution from #396