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.

Now we use 'children' uniformly, in both inlines and blocks,
for child nodes.

This matches the C impl.

Also removed an unused property.

In JS, use 'Text' instead of 'Str'.

In spec, use "plain textual content" instead of "strings."

Don't distinguish fenced, indented.

This reverts commit 4570eb2bff2e1b71fa5b6408abbc69c98ff5ff24.