Introduce Not for GraphNameMatcher and TermMatcher

[mkatychev]

It was often found handy to exclude certain terms or graph names during a filter:

let my_graph_name: SimpleTerm<'static> =
    default_namespace.get_unchecked("#my_graph").into_term();
let ds_without_my_graph: Vec<Spog<SimpleTerm>> = dataset
    .quads_matching(Any, Any, Any, Not(Some(my_graph_name)))
    .collect_quads()
    .unwrap();

[mkatychev]

If unit tests are needed, I'd like to know what module they should go under.

[pchampin]

all tests for (GraphName|Term)Matcher are currently in api/src/term/matcher.rs. Having tests for Not would indeed be nice.

[pchampin]

that's great; thanks