bug#26073: workaround

[Stefan Monnier]

> At least the docstring of `with-slots' and the EIEIO manual both state:
> "Both ‘setf’ and ‘setq’ can be used to set the value of the slot." In
> particular, it doesn't state that `let' can be used for this purpose. So as
> long as `setf' and `setq' continue working, I think it should be OK to
> change the behavior (it should still be documented as incompatible Lisp
> change though).

Right, the interaction with `let` is not documented, but some packages
may still rely on it.  What this means is that fixing bug#26073 for
Emacs-26 may require introducing a second cl-symbol-macrolet so as not
to risk breaking packages using the old semantics.

I think for `master` it's OK to just change cl-symbol-macrolet (and risk
breaking other packages).


        Stefan