Add note about macros

TODO.md

@@ -4,6 +4,14 @@
 
 * cf. https://siraben.dev/2022/03/22/tree-sitter-linter.html write a tree-sitter parser for Schala
 
+* Create a macro system, perhaps c.f. Crystal's?
+    * Macro system should be able to implement:
+        * printf-style variadic arguments
+        * something like the Rust/Haskell `Derive` construct
+        * doing useful things with all variants of an enum 
+            * (e.g. what https://matklad.github.io//2022/03/26/self-modifying-code.html tries to solve)
+
+
 ## Testing
 
 * Make an automatic (macro-based?) system for numbering compiler errors, this should be every type of error