message: Add tests for parameter passing optimisation
  Summary: I thought we might have got this wrong. Turns out we didn't, but for very subtle reasons, so add a test.
diff: @@ -284,6 +284,9 @@ impl<'v> ParametersSpec<Value<'v>> {
  && params.args.is_none()
  && params.kwargs.is_none()
  {
  + // because len == both spec.kinds and spec.positional, there can't be
  + // no_args, *args or **kwargs
  +
  // fast path for saturated length only positional
  for (i, x) in params.pos.iter().enumerate() {
  eval.local_variables.get_slot_at(slots, i).set_direct(*x)
  