Speed up traversing nodes with node visitors

[nicolas-grekas]

NodeTraverser walks the whole tree once per node visitor, and it iterated over the children of each node with foreach ($node as ...), which allocates an ArrayIterator per node and per visitor. It now reads the children array through a new internal Node::getNodes() method. On the Symfony Demo, where six visitors are registered, walking the 21k nodes of its 52 templates takes half the instructions it took, and warming up its Twig cache runs 15% fewer instructions and takes 520ms instead of 566ms.

The traverser doesn't go through getIterator() anymore, so a subclass overriding it to hide children from visitors would be bypassed, and one declaring its own getNodes() would clash; I found none in Twig, Symfony or UX. Calling getArrayCopy() on the iterator returned by getIterator() would avoid the new method, but it saves 13% of the traversal instructions instead of 50%: new \ArrayIterator() and getArrayCopy() both copy the array.

Merge-up notes

To 4.x: the patch applies as is (I ran the 4.x suite with it), the CHANGELOG entry doesn't go to 4.x.