Function eval_async_with_storage_externs 

pub fn eval_async_with_storage_externs<'life0, 'life1, 'life2, 'async_recursion, D>(
    node: &'life0 Node,
    ctx: Arc<dyn CtxStorage>,
    dispatcher: &'life1 D,
    externs: &'life2 (dyn Externs + Sync),
) -> Pin<Box<dyn Future<Output = Result<(), EvalError>> + Send + 'async_recursion>>
where
    D: AsyncDispatcher + ?Sized + 'async_recursion,
    'life0: 'async_recursion,
    'life1: 'async_recursion,
    'life2: 'async_recursion,

eval_async_with_storage + externs registry for call_extern Expr resolution. externs must be Sync so the recursive future stays Send (host executors spawn it across threads).