Use linked list instead of arrays for AST.

Use the same doubly linked node structure that cmark uses.
The primary advantages of this change are (a) simplified code,
especially in the renderers, and (b) elimination of the need for
recursion, so we can render deeply-nested structures without a
stack overflow.

A node walker has also been added, for easy AST traversal.

* Added js/lib/node.js for nodes. Includes a node walker.
* All modules updated to use node structures.
* Regularized position information into pos property.
* Performance is slightly worse than before, but only marginally,
  and no doubt there are more optimizations that can be done.

1 files changed, 1 insertions, 1 deletions

diff --git a/js/bench.js b/js/bench.js
index 77f5025..a5e0d91 100644
--- a/js/bench.js
+++ b/js/bench.js
@@ -17,7 +17,7 @@ suite.add('commonmark.js markdown->html', function() {
   "use strict";
   var doc = new sm.DocParser().parse(contents);
   var renderer = new sm.HtmlRenderer();
-  renderer.renderBlock(doc);
+  renderer.render(doc);
 })
 
 .add('showdown.js markdown->html', function() {