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.