Refactor: Rename TCO module to lowering

The TCO (Tail Call Optimization) module has been renamed to `lowering`.
This change better reflects the module's broader responsibility, which
includes not only TCO but also general AST transformations and
preparation for VM execution.

The `optimize` function has been renamed to `lower` to align with the
module's new name.

src/ast/compiler/mod.rs

@@ -7,7 +7,7 @@ pub mod lambda_collector;
 pub mod macros;
 pub mod optimizer;
 pub mod specializer;
-pub mod tco;
+pub mod lowering;
 pub mod type_checker;
 
 pub use binder::*;
@@ -17,5 +17,5 @@ pub use dumper::*;
 pub use macros::*;
 pub use optimizer::*;
 pub use specializer::*;
-pub use tco::*;
+pub use lowering::*;
 pub use type_checker::*;