parse: cookies should be maybe_unused

They aren't necessarily _doing_ anything. Just waiting to go out of
scope.

parse/fwd.hpp

@@ -10,6 +10,6 @@
 
 namespace cruft::parse {
     namespace enumeration {
-        struct cookie { void release (void) {} };
+        struct [[maybe_unused]] cookie { void release (void) {} };
     }
 }