Weaken access requirements for Element internals

These methods were protected since day 1.
If you only used RootElement, you didn't notice this,
but HHVM did not allow RootElement to invoke your
Element methods, unless you made them public.
hhvm@next requires that RootElement have public
methods, since calling protected interface methods is banned.
This is a breaking change, but users that used sgml-stream
(with or without non-RootElement elements) would not
notice, unless their custom element type has never been rendered.

Author: hershel-theodore-layton

src/Element.hack

@@ -25,7 +25,7 @@ interface Element extends ToSGMLStringAsync, Streamable {
    * Returns the direct children of an Element. If an element has no children,
    * an empty vec must be returned.
    */
-  protected function getChildren()[]: vec<XHPChild>;
+  public function getChildren()[]: vec<XHPChild>;
 
   /**
    * Must return all assigned data- and aria- attributes and all data- and aria-
@@ -34,13 +34,13 @@ interface Element extends ToSGMLStringAsync, Streamable {
    * attribute has a non arraykey value, either an exception must be thrown, the
    * value must be cast to an arraykey, or the value must be filtered out.
    */
-  protected function getDataAndAriaAttributes()[]: dict<string, arraykey>;
+  public function getDataAndAriaAttributes()[]: dict<string, arraykey>;
 
   /**
    * Must return all assigned declared attributes and all declared attributes
    * with a default value. If an attribute has both a default value and an
    * assigned value, the assigned value must be used. Declared aria- and data-
    * attributes must be filtered out.
    */
-  protected function getDeclaredAttributes()[]: dict<string, nonnull>;
+  public function getDeclaredAttributes()[]: dict<string, nonnull>;
 }