Refactor AST substitution logic

This commit refactors the AST substitution logic to correctly handle
nested expansions and improve the inlining of lambda expressions.

The changes include:
- Extracting the core value from potentially expanded AST nodes before
  checking their kind.
- Ensuring that lambda expressions with empty upvalues are correctly
  substituted.
- Adding support for inlining global get expressions as AST
  substitutions.
- Improving the `UsageInfo` collection to correctly track assigned
  values.

src/ast/compiler/optimizer/utils.rs

@@ -143,8 +143,7 @@ impl UsageInfo {
                     self.collect(v);
                 }
             }
-            BoundKind::Define { addr, value, .. } => {
-                self.assigned.insert(*addr);
+            BoundKind::Define { value, .. } => {
                 self.collect(value);
             }
             BoundKind::Expansion { bound_expanded, .. } => {