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get rid of substitution in Case (use lambdas); introduce Match as derived form that involves binders

We no longer abuse empty strings for anonymous binders. Instead, we
now have a data type for binders: a binder is either named or
anonymous.

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

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.

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.

* Use sig instead of sigT: the proof is a Prop after all
* Tweak implicit arguments
* Shorten proof of sigma

This way it behaves better for discrete CMRAs.

I am now also using reification to obtain the indexes corresponding to
the stuff we want to cancel instead of relying on matching using Ltac.