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Since functor instances are just used as combinators, there is really
no need for functors that are not contractive.

This cleans up some ad-hoc stuff and prepares for a generalization
of saved propositions.

It now also contains a non-expansiveness proof.

I have simplified the following CMRA axioms:

  cmra_unit_preservingN n x y : x ≼{n} y → unit x ≼{n} unit y;
  cmra_op_minus n x y : x ≼{n} y → x ⋅ y ⩪ x ≡{n}≡ y;

By dropping off the step-index, so into:

  cmra_unit_preservingN x y : x ≼ y → unit x ≼ unit y;
  cmra_op_minus x y : x ≼ y → x ⋅ y ⩪ x ≡ y;

The old axioms can be derived.

The performance gain seems neglectable, unfortunatelly...

This way it behaves better for discrete CMRAs.

This better seals off their definition. Although it did not give
much of a speedup, I think it is conceptually nicer.

(missed practically everything in the previous commit)

Also, make our redefinition of done more robust under different
orders of Importing modules.

Also do some minor clean up.

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.

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.

* 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.

Also change the definition of agreement slightly, so agree_map can be defined
for any function, and not just non-expansive functions.