This removes the ASDL code generation in favor of handwriting the AST.
The motivations for moving away from the ASDL are:
* CPython compatibility is no longer a goal
* The ASDL grammar isn't as expressive as we would like
* The codegen scripts have a high complexity which makes extensions time
consuming
* We don't make heavy use of code generation (compared to e.g.
RustPython that generates Pyo3 bindings, a fold implementation etc).
We may want to revisit a grammar based code generation in the future,
e.g. by using [ungrammar](https://github.com/rust-analyzer/ungrammar)
