No idea why these aren't resolved automatically, for unary predicates
they do not seem necesarry.

Instead, we have just a construction to create a CMRA from a RA. This
construction is also slightly generalized, it now works for RAs over any
timeless COFE instead of just the discrete COFE.

Also:
* Put tactics and big_ops for CMRAs in a separate file.
* Valid is now a derived notion (as the limit of validN), so it does not have
  to be defined by hand for each CMRA.

Todo:
Make the constructions DRA -> CMRA and RA -> CMRA more uniform.

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.