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/middleend/mod.rs

@@ -1 +1,2 @@
-pub mod crawler;
+pub mod variable_resolution;
+pub mod visit_ast;