ConditionalBuilder allows adding of scoped definitions that break validation

[acl-cqc]

let mut cb = ConditionalBuilder::new(vec![type_row![]; 2], type_row![], usize_t()).unwrap();
let cst = cb.add_constant(Value::from(ConstUsize::new(29)));
let mut case0 = cb.case_builder(0).unwrap();
let c = case0.load_const(&cst);
let case0 = case0.finish_with_outputs([c]).unwrap().node();
let mut case1 = cb.case_builder(1).unwrap();
let c = case1.load_const(&cst);
let case1 = case1.finish_with_outputs([c]).unwrap().node();
let orig = cb.finish_hugr().unwrap();

I'm not really sure whether

1. validation should accept this Hugr; the conditional should be compiled that the first Case should be used for the first tag of the predicate, rather than "the first child"; or
2. ConditionalBuilder should not implement the trait defining add_constant (and all the others - e.g. local functions).

It's not useful to say you have to add all the Cases before the other scoped defns as you then wouldn't be able to use the latter.