feat: lots of optimization

* allocate vm stack on the stack
* use Rc instead of Arc
* optimize recursive attribute set

src/ir.rs

@@ -695,7 +695,9 @@ fn downgrade_attr(attr: ast::Attr, ctx: &mut DowngradeContext) -> Result<Attr> {
         Ident(ident) => Ok(Attr::Str(ident.to_string().into())),
         Str(string) => {
             let parts = string.normalized_parts();
-            if parts.len() == 1 {
+            if parts.len() == 0 {
+                Ok(Attr::Str("".into()))
+            } else if parts.len() == 1 {
                 match parts.into_iter().next().unwrap() {
                     Literal(ident) => Ok(Attr::Str(ident.into())),
                     Interpolation(interpol) => {