Generate empty CtorSaturated, optimize it in codegen instead

[MonoidMusician]

This is useful for other backends where constructing data is cheap and cross-module references are expensive (like Erlang, where top-level declarations need to be functions). It would be difficult to reconstruct what Var reference is really a nullary ctor during codegen, so it is better to keep the constructors and let backend-es optimize the nullary case to its liking.

There's tiny differences in optimizations in the snapshots:

• CaseLeafTco looks like it inlined last, which looks like an improvement to me (no more intermediate Maybe result)
• Fusion01/Fusion02 inlined toUnfoldable a bit

[natefaubion]

I think the codegen changes happen because these functions now analyze as evaluating to a KnownNeutral, where the previous Var encoding made it opaque. I think that's desirable.

[natefaubion]

cc @anttih This will affect the scheme backend likely.