New Feature: NodeIterator

[colinodell]

Description

The NodeWalker we use today works well in some cases but is quite inefficient in other cases for the following reasons:

1. Blocks and container nodes are visited twice (upon entering and leaving), but in most cases we only care about visiting them once
2. It requires wrapping the node in a NodeWalkerEvent object
3. It's not easy to skip certain parts of the AST

I'd therefore like to add a NodeIterator which offers the same simplified approach to looping through AST nodes but with the following features:

1. Each node should be visited exactly once
2. Each node object should be returned directly - no intermediate wrapping classes
3. It should support skipping branches of the tree which only contain inlines (in cases where we only care about blocks)
4. It should make AST iteration easier for developers

In the best-case scenario, this should result in AST iteration times being cut in half since each node is only visited once instead of twice.

Example

Here's what the usage would look like in practice - notice how much of the boilerplate goes away!

-$walker = $document->walker();
-while ($event = $walker->next()) {
-    if (! $event->isEntering()) {
-        continue;
-    }
-
-    $node = $event->getNode();
+foreach ($document->iterator() as $node) {
     if ($node instanceof Link) {
          // TODO: do something
     }
 }