Automate AST generation

[ForNeVeR]

In #678, we've got a manual AST generator for verification purposes.

For now, it is fully manually written, and I believe this is far from productive.

Let's consider automating it somehow. Some basic ideas:

1. Get an AST visitor and use it instead of manual enumeration of all nodes
2. Write a source gen
3. Convert to JSON and then make some basic string or JSON tree transforms

[kant2002]

I think AST nodes should have information about their range and source file of origin.

[ForNeVeR]

They should, but this is separate topic I believe. Let's say it's currently anchored by #526.