perf: make the most of streamed rendering

src/drop/block-drop.ts

@@ -3,10 +3,14 @@ import { Drop } from './drop'
 export class BlockDrop extends Drop {
   constructor (
     // the block render from layout template
-    private superBlockRender: () => Iterable<string> = () => ''
+    private superBlockRender: () => Iterable<any> = () => ''
   ) {
     super()
   }
+  /**
+   * Provide parent access in child block by
+   * {{ block.super }}
+   */
   public super () {
     return this.superBlockRender()
   }