Refactor: Move utils to separate module

Moves `PathTracker` and `UsageInfo` to the `utils` module to improve
code organization.

src/ast/compiler/optimizer/mod.rs

@@ -7,4 +7,5 @@ pub mod utils;
 pub use engine::Optimizer;
 pub use folder::Folder;
 pub use inliner::Inliner;
-pub use substitution_map::{SubstitutionMap, UsageInfo};
+pub use substitution_map::SubstitutionMap;
+pub use utils::{PathTracker, UsageInfo, flatten_tuple};