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/environment.rs

@@ -134,9 +134,7 @@ impl Environment {
         let mut purity = self.global_purity.borrow_mut();
 
         let idx = values.len() as u32;
-        let identity = Rc::new(NodeIdentity {
-            location: SourceLocation { line: 0, col: 0 },
-        });
+        let identity = NodeIdentity::new(SourceLocation { line: 0, col: 0 });
         names.insert(Symbol::from(name), (idx, identity));
         types.insert(idx, ty);
         purity.insert(idx, func.purity);
@@ -167,9 +165,7 @@ impl Environment {
         let mut purity = self.global_purity.borrow_mut();
 
         let idx = values.len() as u32;
-        let identity = Rc::new(NodeIdentity {
-            location: SourceLocation { line: 0, col: 0 },
-        });
+        let identity = NodeIdentity::new(SourceLocation { line: 0, col: 0 });
         names.insert(Symbol::from(name), (idx, identity));
         types.insert(idx, ty);
         purity.insert(idx, Purity::Pure);