Factory function that creates a closure scope around a Thenable-callback.
Returns a function that can be passed to a Thenable that will resume a
Context lookup once the Thenable resolves with new data, adding that new
data to the lookup stack.
function getWithResolvedData(ctx, cur, down) {
    return function(data) {
      return ctx.push(data)._get(cur, down);
    };
  }