Panic using global names in roto script

[DRiKE]

Based on the big-overhaul branch, a4002c5

Not quite sure whether I'm using this as intended, but including any of the uppercase global names as defined in fn typechecker::types::globals(..) causes a panic a la

did not find Var { scope: ScopeRef(None), kind: Explicit(Identifier(SymbolU32 { value: 5 })) } in { ...

#[test]
fn issue_5() {
    let rt = Runtime::basic().unwrap();

    let s = r#"
        filter-map main() {
            define {
                safi = Safi.Unicast;
            }
            apply {
                //  this panics at codegen/mod.rs:634:
                //  "did not find Var { ... }
                if safi == UNICAST {

                // this works however:
                //if safi == Safi.Unicast {

                    reject
                }
                accept

            }
        }
    "#;

    let _p = compile(s, rt);
}