`equal` is not error-free

`equal` is not error-free

* lisp/emacs-lisp/byte-opt.el (side-effect-free-fns)
(side-effect-and-error-free-fns)
(byte-compile-side-effect-and-error-free-ops)
(byte-compile-side-effect-free-ops):
Demote `equal` and `equal-including-properties` from error-free to
merely side-effect-free since they may in fact signal error on
circularity.