Refactor: Improve optimizer and DCE

This commit introduces several improvements to the AST optimizer and
dead code elimination (DCE) logic:

- **Address Mapping:** The `Get` and `Set` operations now correctly map
  local slots and global indices using the substitution map, ensuring
  that remapped variables are handled properly.
- **Parameter Slot Mapping:** Parameters are now explicitly mapped to
  their correct slots within the `Lambda` node, preventing potential
  slot reassignments in the function body.
- **Pure Function Analysis:** The `is_pure` function has been refined to
  accurately identify pure expressions, including calls to pure
  functions and stateless closures.
- **Dead Code Elimination:** DCE logic in `BoundKind::Block` has been
  enhanced to remove unused local definitions and pure, non-essential
  expressions more effectively. Global DCE is also improved.
- **Constant Folding:** `try_fold_pure` is now more robust in folding
  pure function calls with constant arguments, reducing redundant
  computations.
- **Beta Reduction:** `try_beta_reduce` has been updated to avoid
  reducing if the body contains `DefLocal`, ensuring correctness.
- **Global Purity Tracking:** A `global_purity` map is introduced to
  track the purity of global values, enabling better optimization of
  global accesses.
- **Inlinable Value Check:** The `is_inlinable_value` check now
  correctly identifies stateless closures for inlining.
- **Optimization Level:** The default `optimization_level` is set to 2
  to enable more aggressive optimizations.

src/ast/environment.rs

@@ -91,7 +91,7 @@ impl Environment {
             function_registry: Rc::new(RefCell::new(HashMap::new())),
             monomorph_cache: Rc::new(RefCell::new(HashMap::new())),
             debug_mode: false,
-            optimization_level: 0,
+            optimization_level: 2,
         };
         env.register_stdlib();
         env