macros, vm, bytecode: add #[handler] await-style primop macro

fix-compiler/src/ir/downgrade.rs

@@ -466,7 +466,7 @@ impl<'id: 'ir, 'ir, Ctx: DowngradeContext<'id, 'ir>> Downgrade<'id, 'ir, Ctx> fo
             body: GhostRoIrRef<'id, 'ir>,
         }
 
-        let (ret, thunks) = ctx.with_thunk_scope(|ctx| {
+        let (ret, thunks) = ctx.with_thunk_scope(|ctx| -> Result<Ret> {
             let (param, body) = match raw_param {
                 ast::Param::IdentParam(id) => {
                     let param_sym = ctx.intern_string(id.to_string());
@@ -506,7 +506,7 @@ impl<'id: 'ir, 'ir, Ctx: DowngradeContext<'id, 'ir>> Downgrade<'id, 'ir, Ctx> fo
                 }
             };
 
-            Result::Ok(Ret { param, body })
+            Ok(Ret { param, body })
         });
         let Ret { param, body } = ret?;