Detect cyclic DAGs in validation

[acl-cqc]

As per #228, this test creates a cyclic DAG but validation is "happy":

   #[test]
    fn test_cyclic() -> Result<(), HugrError> {
        let mut h = Hugr::new(NodeType::pure(ops::DFG {
            signature: FunctionType::new(type_row![QB_T], type_row![QB_T]),
        }));
        let input = h.add_op_with_parent(h.root(), ops::Input::new(type_row![QB_T]))?;
        let output = h.add_op_with_parent(h.root(), ops::Output::new(type_row![QB_T]))?;
        let cx = h.add_op_with_parent(h.root(), cx_gate())?;
        h.connect(input, 0, cx, 0)?;
        h.connect(cx, 0, output, 0)?;
        h.connect(cx, 1, cx, 1)?;
        h.validate().unwrap(); // TODO this should raise an error
        Ok(())
    }

Instead we should probably get a ValidationError::NotABoundedDag (or perhaps a new error, but anything cyclic clearly isn't any sort of DAG bounded or otherwise)