Render back to markdown

[Ericson2314]

It would be nice to render the AST back to markdown. This has two purposes:

• Test round trips. If we render then parse and AST, and we should get the same AST we started with.

• In conjunction with Directly construsting the AST #131 not all ASTs will have come from the parser, so rendering an AST to markdown is a useful way to programmatically create markdown.

[kristapsdz]

I see your point, but it's unlikely I'll do this myself. I'm happy to review and merge patches for this, however.