ENH: convert syntax tree to JSON

[johnyf]

These changes serialize the abstract syntax tree as JSON.

[ahelwer]

@johnyf do you think something like this would be better implemented by sticking PPX macros onto our data structures?

[johnyf]

Not sure about PPXs. It may be simpler to have an implementation as a function.

[ahelwer]

This seems like useful functionality I'd like to play around with, would you be able to rebase and fix the conflicts?

P.S. what does ENH stand for?

[johnyf]

I will try to rebase the changes. The prefix "ENH" stands for "enhancement" (in the sense of a new feature), and is among acronyms listed in numpy's documentation https://numpy.org/doc/stable/dev/development_workflow.html#writing-the-commit-message.