Move variable resolution to middleend

- Introduce AST-based visitor and resolver in middleend
- Update guard field and parser to use AstNode<Expr>
- Remove backend variable_resolution and adjust exports
- Expose middleend in the library

src/frontend/parser.rs

@@ -252,11 +252,10 @@ impl Parser {
             end_position: end_position.end_position,
         };
 
-        let mut guard: Option<Box<Expr>> = None;
+        let mut guard: Option<Box<AstNode<Expr>>> = None;
         if self.peek().token_type == TokenType::LeftBrace {
             self.consume(TokenType::LeftBrace, "Expected '{' after guard expression")?;
-            println!("ho trovato una guradia");
-            guard = Some(Box::new(self.expression()?.node.clone()));
+            guard = Some(Box::new(self.expression()?));
             self.consume(TokenType::RightBrace, "Expected '}' after guard expression")?;
         }