Contrary to destruct_conj from Program.

For consistency's sake.

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.