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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.

This way it behaves better for discrete CMRAs.

This fix removes superflous white space in pretty printing. For example
✓{S n} x was pretty printed incorrectly as ✓{(S n)} x.

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

Since Coq 8.4 did not backtrack on eauto premises, we used to ensure
that hints like

  Hint Extern 0 (?x ≡{_}≡ ?y) => reflexivity.

were not used for goals involving evars by writing ?x ≡{_}≡ ?y instead
of _ ≡{_}≡ _.

This seems to be a legacy issue that no longer applies to Coq 8.5, so
I have removed these restrictions making these hints thus more powerful.

Also do some minor clean up.

Instead, I separate it into a total function and a predicate
describe whether the action is allowed or not. This has some
advantages:

* It is much easier to deal with total functions and predicates
  in Coq than with functions into option.
* Already existing functions do not need to be wrapped. Instead,
  when using a local update you end up with a sensible side
  condition as a Coq Prop.
* The definition of local updates (and all CMRA instances) no
  longer depend on option.

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.

* Insert and singleton operation.
* Identity element.
* Non-expansiveness and properness of insert and singleton.
* Frame preserving updates.
* Functoriality.