Add PrimValue::Graph

[acl-cqc]

Possibly unless we drop "Graph" types from core, which we might do according to outcome of #285.

Again according to #285, this might be as simple as

struct Graph(Hugr);

impl Graph {

    fn new(h: Hugr) -> Self {
        assert!(h.is_dfg());
        Self(h)
    }
}

(if monomorphic) whereas if polymorphic it might be a bit more complex