Serialize the store contents to a string
Examples
store <- rdf_store_new()
rdf_store_load(store, '<http://example.org/s> <http://example.org/p> "v" .', "ntriples", NULL)
rdf_store_dump(store, "turtle")
#> [1] "<http://example.org/s> <http://example.org/p> \"v\" .\n"
