All Streamable implementations are valid a XHPChild

This should have always been the case, see `<p>{$streamble}</p>`.

Author: hershel-theodore-layton

src/Streamable.hack

@@ -1,12 +1,14 @@
 /** sgml-stream-interfaces is MIT licensed, see /LICENSE. */
 namespace HTL\SGMLStreamInterfaces;
 
+use type XHPChild;
+
 /**
  * Classes which desire to render in Snippets. Implementations may decide to
  * throw a UseAfterRenderException when it is asked to render itself multiple
  * times.
  */
-interface Streamable {
+interface Streamable extends XHPChild {
   /**
    * Must insert its content into $stream while the call has not yet returned
    * or thrown. Implementations may not insert content after the method has