Drupal\Core\Template and twig.engine docs, coding standards, and unused code cleanup

Looking great overall! Few nits:

1. +++ b/core/lib/Drupal/Core/Template/TwigExtension.php
   @@ -50,15 +64,19 @@ public function getNodeVisitors() {
      }
    }
   -
   

   Nit: Great that we're removing the extra blank line from EOF here but we should also add a blank line before the end of the class declaration per https://drupal.org/node/608152#indenting.

2. +++ b/core/lib/Drupal/Core/Template/TwigNodeVisitor.php
   @@ -26,12 +26,7 @@ function enterNode(\Twig_NodeInterface $node, \Twig_Environment $env) {
   -   * Implements Twig_NodeVisitorInterface::leaveNode().
   -   *
   -   * We use this to inject a call to render_var -> twig_render_var()
   -   * before anything is printed.
   -   *
   -   * @see twig_render
   +   * {@inheritdoc}
   

   Can we move the "We use this" comment inside the leaveNode method? I've found it really useful to know what this code is doing :)

3. +++ b/core/lib/Drupal/Core/Template/TwigNodeVisitor.php
   @@ -47,10 +42,10 @@ function leaveNode(\Twig_NodeInterface $node, \Twig_Environment $env) {
   -   * Implements Twig_NodeVisitorInterface::getPriority().
   +   * {@inheritdoc}
       */
      function getPriority() {
   -    // We want to run before other NodeVisitors like Escape or Optimizer
   +    // We want to run before other NodeVisitors like Escape or Optimizer.
   

   This hunk can be removed since this will be taken care of in #2221535: TwigNodeVisitor's priority is too early and breaks some filters and macros.

Damn TwigReference had its claws in everywhere! That code removal can be a separate issue if needed but other than that this all looks like good cleanup to me and I don't see much if any conflicts happening (other than the autoescape patch but should be easy to resolve those).

10: 2212309-twig-engine-doc-cleanup-10.patch queued for re-testing.

+++ b/core/lib/Drupal/Core/Template/TwigNodeVisitor.php
@@ -47,9 +44,10 @@ function leaveNode(\Twig_NodeInterface $node, \Twig_Environment $env) {
-   * {@inheritdoc}
+   * Implements Twig_NodeVisitorInterface::getPriority().

This needs to be reverted.

Great, basically the same as #6 now. Thanks! +1 to RTBC.

Committed and pushed to 8.x. Thanks!