Consider operating on quads/datasets for IOService

[acoburn]

The I/O service currently operates only on Triple and Graph objects. This makes sense for triple-based RDF serializations, but it excludes any possible support for quad-based serializations (TriG, N-Quads or JSON-LD (with dataset support)). Elsewhere, we have successfully converted interfaces from Triple/Graph objects to Quad/Dataset objects, and this would, arguably, make this more consistent across the various services.