Now we substitute as far into the term as we can. This is to deal
with terms that contain Coq variables.

It is based on type classes and can it be tuned by providing
instances, for example, instances can be provided to mark that
certain expressions are closed.

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 changes are probably necessary because rewrite now tries harder not to
instantiate evars, which it always said it would not do.

It now turns setoid equalities into Leibniz equalities when possible,
and substitutes those.