minor refactor

examples/reuse_scope.rs

@@ -5,12 +5,7 @@ fn main() {
     let mut engine = Engine::new();
     let mut scope: Scope = Vec::new();
 
-    if !engine
-        .eval_with_scope::<()>(&mut scope, "let x = 4 + 5")
-        .is_ok()
-    {
-        assert!(false);
-    }
+    assert!(engine.eval_with_scope::<()>(&mut scope, "let x = 4 + 5").is_ok());
 
     if let Ok(result) = engine.eval_with_scope::<i64>(&mut scope, "x") {
         println!("result: {}", result);