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/substitution_map.rs

@@ -143,6 +143,7 @@ impl UsageInfo {
     }
 }
 
+#[derive(Default)]
 pub struct SubstitutionMap {
     pub values: HashMap<Address, Value>,
     pub ast_substitutions: HashMap<Address, AnalyzedNode>,
@@ -155,15 +156,7 @@ pub struct SubstitutionMap {
 
 impl SubstitutionMap {
     pub fn new() -> Self {
-        Self {
-            values: HashMap::new(),
-            ast_substitutions: HashMap::new(),
-            slot_mapping: HashMap::new(),
-            assigned: HashSet::new(),
-            next_slot: 0,
-            used: HashSet::new(),
-            captured_slots: HashSet::new(),
-        }
+        Self::default()
     }
 
     pub fn add_ast_substitution(&mut self, addr: Address, node: AnalyzedNode) {