Provide a reference implementation and notes for implementors

[jrudolph]

This is very important: real problems will only surface once someone starts to use the AST.

Questions that will come up:

• How should the basic APIs interact with the AST?
• Which AST kind should methods accept (the raw one or the full-featured one from this implementation)?
• How is interaction between two json libraries going to work?

etc.

[mdedetrich]

Should look at this once the design is a bit more concrete. Think with all of the attention we are getting now, the design will probably be revisited.