Add support for reading RDF*

[csi-amolpawar]

As a serd api user, Need a support for RDF* data present in the turtle/trig file

  << <person3> <dislike> "jazz" >> <weight> 9 .
  << <person3> <dislike> "theatre" >> <weight> 5 .
  << <person3> <dislike> "opera" >> <weight> 10 .

See Reference https://www.w3.org/2022/08/rdf-star-wg-charter/

[drobilla]

Not likely unless somebody funds it, to be honest. I have little to no incentive for this personally, and RDF* seems like it raises a lot of questions that would require design changes, especially in a library designed around streaming that exploits the relatively simple structure of RDF data, has no concept of statement identification, and so on.

Perhaps it's doable without too much trouble, but would certainly be for the next-next major version at least.

[drobilla]

I am reworking the statement storage for the next version, and there's other reasons why having statement IDs might be the way to go internally, so I'll at least have an idle think about whether RDF* is another reason to do so.

That's as far as it will go in terms of things actually in releases until the specs are finalized, but a long-lived branch might be possible until then.