Add note about typestates

TODO.md

@@ -42,6 +42,7 @@
 ## Language Syntax
 
 * the `type` declaration should have some kind of GADT-like syntax
+* syntactic sugar for typestates? (cf. https://rustype.github.io/notes/notes/rust-typestate-series/rust-typestate-part-1.html )
 * use `let` sigil to indicate a variable in a pattern explicitly:
 
 ```