Simple implementation of SPARQL queries

[jamescheney]

This is for discussion/testing fo rthe moment. Some problems to potentially resolve before merging:

• [ ] rdf and some other libraries are added as hard Links dependencies. It would be good to have a separate "driver" for the SPARQL querying interface, like for the databases.
• [ ] Currently a single function is exposed which takes three arguments: a "base IRI" (needed only if the result is a graph, AFAICT), the URI of the SPARQL endpoint, and the SPARQL query to actually run. More functions for example to cater to the common case where the base isn't needed, and to allow for different flavors of SPARQL queries, such as ASK (returns boolean) or CONSTRUCT (returns graph), might be nice. Currently only SELECT (return list of variable-value bindings) is supported.
• [ ] The code currently throws away type/structure information about the query results and just converts everything to strings. WE could retain the structure of the returned RDF values/terms in Links instead.

[jamescheney]

In today's meeting I think we agreed that this can be merged, with the todos above split into a separate issue along with a reminder to avoid/revisit the dependency on LWT when LWT goes away/is subsumed by other capabilities of OCaml 5.

[jamescheney]

I think @vcgalpin agreed to try this branch out with some larger Links programs and @wricciot agreed to look over the code.

[jamescheney]

Light is green, trap is clean.