I planned to use them to simplify wsat_le, but it did not turn out to be simpler.

This is a workarround, see: https://github.com/math-comp/math-comp/issues/18

Sadly, timelessness of many connectives is still proved in the model.

Now we use a specific notation (⊑) to whose arguments we interpret
in uPred_scope. This makes notations much more concise.

Also introduce a notion for valid to be timeless.

* Remove the order from RAs, it is now defined in terms of the ⋅ operation.
* Define ownership using the step-indexed order.
* Remove the order also from DRAs and change STS accordingly. While doing
  that, I changed STS to no longer use decidable token sets, which removes the
  requirement of decidable equality on tokens.