Investigate Halide rule validation

[ajpal]

See #160 for context.
With just -1, 0, and 1 in the cvecs, this will crash:
With additional constants added, it works.
My guess is there is a bug in the rule validation that is avoided by putting more stuff in the cvec, but we should spend some more time getting to the bottom of this.

#[test]
    fn div_rules() {
        let lang = Workload::new(&["const", "var", "(uop expr)", "(bop expr expr)"]);
        let rules4 = Pred::run_workload(
            lang.clone()
                .iter_metric("expr", ruler::enumo::Metric::Atoms, 4)
                .plug_lang(
                    &Workload::new(&["-1", "0", "1"]),
                    &Workload::new(&["a", "b", "c"]),
                    &Workload::new(&["-"]),
                    &Workload::new(&["+", "-", "*", "/", "min", "max"]),
                ),
            Ruleset::default(),
            Limits::default(),
        );
        rules4.pretty_print();
        let wkld = lang
            .iter_metric("expr", ruler::enumo::Metric::Atoms, 5)
            .plug_lang(
                &Workload::new(&["-1", "0", "1"]),
                &Workload::new(&["a"]),
                &Workload::new(&["-"]),
                &Workload::new(&["+", "-", "*", "/", "min", "max"]),
            );
        Pred::run_workload(wkld, rules4, Limits::default());
    }