Add a var! macro. (#548)(#550) r=emily 
Add a var! macro. (#548)(#550) r=emily 


ncalexan/datomish,ncalexan/mentat,mozilla/mentat,ncalexan/mentat,ncalexan/mentat,ncalexan/mentat,mozilla/mentat,ncalexan/mentat,mozilla/mentat,ncalexan/datomish,ncalexan/mentat,mozilla/mentat,mozilla/mentat,mozilla/mentat