Investigate case semantics

[flupe]

I added a transformation on the treeless syntax that makes all cases exhaustive, even when the branches are unreachable.

However, what used to be the "default" return value for the case is bound in a let binding prior to the match.

Is this problematic?

What is the whcbv semantics for let bindings? If we bind a box in a let binding, will it fail before we reach the case?