Refactor _ast module for clarity by youknowone · Pull Request #7031 · RustPython/RustPython

- Extract singleton_node_to_object() helper for operator/context nodes
- Rename PY_COMPILE_FLAG_AST_ONLY to PY_CF_ONLY_AST
- Rename populate_match_args_and_attributes to populate_repr
- Set _attributes in impl_base_node! no-args variant
- Simplify ast_reduce with .is_some() instead of drop(value)
- Remove _ prefix from used parameters across ast/ files
- Fix slot_new comment to explain why slot_init is called