change is_true and is_false to work properly with unknowns

this means we *always* do the checks for both `var` and `-var`, and that
you can only really "trust" `is_true` and `is_false` if they return
true, otherwise you'd have to use `is_unknown` as well.

rust/solve/src/dpll.rs

}
fn is_true(vars: &BoundVars, var: Var) -> bool {
    vars.contains(&var) || !vars.contains(&-var)
    let t = vars.contains(&var);
    let nt = vars.contains(&-var);
    if t || nt {
        t || !nt
    } else {
        false
    }
}
#[test]
    assert!(is_true(vars, -2));
    assert!(!is_true(vars, -1));
    assert!(!is_true(vars, 2));
    assert!(!is_true(vars, 3));
    assert!(!is_true(vars, -3));
}
fn is_false(vars: &BoundVars, var: Var) -> bool {
    !vars.contains(&var) || vars.contains(&-var)
    let t = vars.contains(&var);
    let nt = vars.contains(&-var);
    if t || nt {
        !t || nt
    } else {
        false
    }
}
#[test]
    assert!(is_false(vars, 2));
    assert!(!is_false(vars, 1));
    assert!(!is_false(vars, -2));
    assert!(!is_false(vars, 3));
    assert!(!is_false(vars, -3));
}
fn is_unknown(vars: &BoundVars, var: Var) -> bool {