AST lives in `ast` package, that relies significantly on `ast-macros` to generate boilerplate.
Additional `macro-utils` library was split out from `shapely-macros` and `ast-macros`.
The implementation was contributed by @wdanilo , I basically just did some refactoring, documenting and testing.
### Important Notes
* AST is known to be incomplete structurally, finishing it is #336
* AST is missing a number of necessary functions, some of them explicitly marked as FIXME in the code, finishing them is #338
* while I have written some tests, they are not yet part of CI — I want to this smart way (i.e. allowing tests that rely on parser), it is tracked as #340
* AST JSON serialization is incompatible with Scala, solving this is #297
* there is some non-deterministic issue with CI on Windows — I need to look into this closer but it seems to not be related to any Rust parts
