Refactor: Implement block scoping and statement separation

This commit introduces fundamental changes to Myc's language structure
to enhance type safety and scoping clarity.

Key changes include:
- Introducing local scopes for `do` blocks.
- Separating statements (`def`, `assign`) from expressions, where
  statements now have no return value and are only allowed within
  blocks.
- A block now consists of multiple statements followed by a single final
  expression.
- Stricter validation rules are enforced, such as disallowing
  redefinition of local symbols in the same scope while allowing
  shadowing of outer symbols.
- Compiler errors are generated for statements used in expression
  contexts.

The implementation involved modifications to the AST, parser, binder
(scope-stack), and various compiler passes, along with VM runtime
optimizations.

examples/object_records.myc

@@ -1,5 +1,5 @@
-;; Benchmark: 1.4us
-;; Benchmark-Repeat: 1473
+;; Benchmark: 1.4us
+;; Benchmark-Repeat: 1473
 ;; Output: [150 130 "Insufficient funds" 130]
 
 ; ---------------------------------------------------------
@@ -20,13 +20,12 @@
         :balance (fn [] balance)
         
         ; Method: Deposit money
-        :deposit (fn [amount] 
-          (assign balance (+ balance amount)))
+        :deposit (fn [amount] (do (assign balance (+ balance amount)) balance))
         
         ; Method: Withdraw money with validation
         :withdraw (fn [amount]
           (if (>= balance amount)
-            (assign balance (- balance amount))
+            (do (assign balance (- balance amount)) balance)
             "Insufficient funds"))
       }
     )))