more documentation of unintuitive packed struct solution

src/step.rs

@@ -28,6 +28,7 @@ pub fn inc_step_counter_and_check_limit(&mut self, n: u64) -> EvalResult<'tcx, (
 
     /// Returns true as long as there are more things to do.
     pub fn step(&mut self) -> EvalResult<'tcx, bool> {
+        // see docs on the `Memory::packed` field for why we do this
         self.memory.clear_packed();
         self.inc_step_counter_and_check_limit(1)?;
         if self.stack.is_empty() {