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/lib.rs

@@ -1,3 +1,4 @@
 pub mod backend;
 pub mod common;
 pub mod frontend;
+pub mod middleend;