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/dumper.rs

@@ -101,9 +101,10 @@ impl Dumper {
                 if !captured_by.is_empty() {
                     for capturer in captured_by {
                         self.write_indent();
+                        let loc = capturer.location.unwrap_or(crate::ast::types::SourceLocation { line: 0, col: 0 });
                         self.output.push_str(&format!(
                             "- Capturer: Lambda at line {}, col {}\n",
-                            capturer.location.line, capturer.location.col
+                            loc.line, loc.col
                         ));
                     }
                 }