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/vm.rs

@@ -1,5 +1,5 @@
 use crate::ast::compiler::bound_nodes::{Address, AnalyzedNode, BoundKind};
-use crate::ast::compiler::tco::ExecNode;
+use crate::ast::compiler::lowering::ExecNode;
 use crate::ast::nodes::Node;
 use crate::ast::types::{Object, Value};
 use std::any::Any;