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This way we avoid many one-off indexes and no longer need special cases for
index 0 in many definitions. For example, the definition of the distance
relation on option and excl has become much easier. Also, uPreds no longer need
to hold at index 0.

In order to make this change possible, we had to change the notions of
"contractive functions" and "chains" slightly.

Thanks to Aleš Bizjak and Amin Timany for suggesting this change and to help
with the proofs.

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.