restored IqRdf-based serializations, including RDF/XML

still simple to parse (cf. 2a1074f7d7d2d30a82c048683db9594f6cbcdf2f),
but now line-based rather than CSV-style (mostly due to IqRdf
limitations - cf. e5c7465efc95a2b8ca84b29a2055514dd22a9810 - but that's
alright)