Process custom element connectedCallback immediately during parsing

Fixes WICG/webcomponents#551 by ensuring that
insertions into the DOM trigger connectedCallback immediately, instead
of putting the callback reaction on the the backup element queue and
letting it get triggered at the next microtask checkpoint. This means
connectedCallback will generally be invoked when the element has zero
children, as expected, instead of a random number depending on when the
next custom element is seen.

source

@@ -103570,31 +103570,39 @@ dictionary <dfn>StorageEventInit</dfn> : <span>EventInit</span> {
   in a given namespace, the user agent must run these steps:</p>
 
   <ol>
-
    <li><p>Let the <var>adjusted insertion location</var> be the <span>appropriate place for
    inserting a node</span>.</p></li>
 
-   <li><p><span>Create an element for the token</span> in the given namespace, with the intended
+   <!-- this insertion stuff should be cleaned up: https://github.com/whatwg/html/issues/1706 -->
+
+   <li><p>Let <var>element</var> be the result of <span data-x="create an element for the
+   token">creating an element for the token</span> in the given namespace, with the intended
    parent being the element in which the <var>adjusted insertion location</var> finds
    itself.</p></li>
 
    <li>
+    <p>If it is possible to insert <var>element</var> at the <var>adjusted insertion location</var>,
+    then:</p>
+
+    <ol>
+     <li><p>Push a new <span>element queue</span> onto the <span>custom element reactions
+     stack</span>.</li>
 
-    <p>If it is possible to insert an element at the <var>adjusted insertion
-    location</var>, then insert the newly created element at the <var>adjusted insertion
-    location</var>.</p>
+     <li><p>Insert <var>element</var> at the <var>adjusted insertion location</var>.</p></li>
 
-    <p class="note">If the <var>adjusted insertion location</var> cannot accept more
-    elements, e.g. because it's a <code>Document</code> that already has an element child, then the
-    newly created element is dropped on the floor.</p>
+     <li><p>Pop the <span>element queue</span> from the <span>custom element reactions stack</span>,
+     and <span>invoke custom element reactions</span> in that queue.</p></li>
+    </ol>
 
+    <p class="note">If the <var>adjusted insertion location</var> cannot accept more
+    elements, e.g. because it's a <code>Document</code> that already has an element child, then
+    <var>element</var> is dropped on the floor.</p>
    </li>
 
-   <li><p>Push the element onto the <span>stack of open elements</span> so that it is the new
+   <li><p>Push <var>element</var> onto the <span>stack of open elements</span> so that it is the new
    <span>current node</span>.</p></li>
 
-   <li><p>Return the newly created element.</p></li>
-
+   <li><p>Return <var>element</var>.</p></li>
   </ol>
 
   <p>When the steps below require the user agent to <dfn>insert an HTML element</dfn> for a token,