Simplify our code generator

[phisiart]

I've been experimenting with rewriting our astgen code generator with jinja2 templates.

[phisiart]

Some progress has been made to simplify the logic of our astgen code generator. I also attempted to convert some parts into jinja2 templates and evaluate their readability.

A critical observation is that AI's ability to work on a codebase is related to its complexity. After simplifying some parts of the current C++ based code generator, AI becomes able to port them to jinja2. Therefore, I'd like to remain focused on logic simplification for now.