Merge branch '1.x' into 2.x

* 1.x:
  Add Parser typehint
  allowed Twig\NodeVisitor\NodeVisitorInterface::leaveNode() to return "null" instead of "false" (same meaning)

src/NodeVisitor/AbstractNodeVisitor.php

@@ -17,6 +17,8 @@ use Twig\Node\Node;
 /**
  * Used to make node visitors compatible with Twig 1.x and 2.x.
  *
+ * To be removed in Twig 3.1.
+ *
  * @author Fabien Potencier <fabien@symfony.com>
  */
 abstract class AbstractNodeVisitor implements NodeVisitorInterface
@@ -41,7 +43,7 @@ abstract class AbstractNodeVisitor implements NodeVisitorInterface
     /**
      * Called after child nodes are visited.
      *
-     * @return Node|false The modified node or false if the node must be removed
+     * @return Node|false|null The modified node or null if the node must be removed
      */
     abstract protected function doLeaveNode(Node $node, Environment $env);
 }