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/types.rs

@@ -139,6 +139,16 @@ impl RecordLayout {
             return layout.clone();
         }
 
+        if fields.is_empty() {
+            let layout = std::sync::Arc::new(RecordLayout {
+                fields: vec![],
+                fmap: vec![],
+                min_idx: 0,
+            });
+            reg.insert(fields, layout.clone());
+            return layout;
+        }
+
         let mut min_idx = u32::MAX;
         let mut max_idx = 0;
         for (k, _) in &fields {