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/common/base_value.rs

@@ -314,7 +314,7 @@ pub struct LoxFunction {
     pub return_type: Option<String>,
     pub body: AstNode<Stmt>,
     pub closure: Option<EnvironmentStack<BaseValue>>,
-    pub guard: Option<Box<Expr>>,
+    pub guard: Option<Box<AstNode<Expr>>>,
 }
 
 impl LoxFunction {
@@ -322,7 +322,7 @@ impl LoxFunction {
         parameters: Vec<(String, String)>,
         body: AstNode<Stmt>,
         closure: Option<EnvironmentStack<BaseValue>>,
-        guard: Option<Box<Expr>>,
+        guard: Option<Box<AstNode<Expr>>>,
     ) -> Self {
         LoxFunction {
             parameters,