Refactor NodeIdentity to use unique IDs

The `NodeIdentity` struct has been refactored to use a unique `id` field
generated by an atomic counter. This ensures that each `NodeIdentity`
instance is distinct, even if it has the same `SourceLocation`. The
`location` field is now optional, allowing for anonymous nodes.

This change improves the reliability of identity comparisons and
provides a more robust way to manage AST node identities.

src/ast/compiler/macros.rs

@@ -717,9 +717,7 @@ mod tests {
             Symbol::from("*"),
             (
                 0,
-                Rc::new(crate::ast::types::NodeIdentity {
-                    location: crate::ast::types::SourceLocation { line: 0, col: 0 },
-                }),
+                crate::ast::types::NodeIdentity::new(crate::ast::types::SourceLocation { line: 0, col: 0 }),
             ),
         );
         let globals = Rc::new(RefCell::new(global_names));