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.

src/ast/nodes.rs

@@ -96,7 +96,8 @@ pub enum UntypedKind {
         lambda: Box<Node<UntypedKind>>,
     },
     Block {
-        exprs: Vec<Node<UntypedKind>>,
+        statements: Vec<Node<UntypedKind>>,
+        result: Box<Node<UntypedKind>>,
     },
     Tuple {
         elements: Vec<Node<UntypedKind>>,