Refactor Optimizer to use Folder

Extract common AST manipulation and folding logic into a new Folder
struct. This improves code organization and reusability. The Optimizer
now delegates these tasks to the Folder.

src/ast/compiler/optimizer/mod.rs

@@ -1,5 +1,7 @@
-pub mod optimizer;
+pub mod engine;
+pub mod folder;
 pub mod substitution_map;
 
-pub use optimizer::Optimizer;
+pub use engine::Optimizer;
+pub use folder::Folder;
 pub use substitution_map::{SubstitutionMap, UsageInfo};