The huge benefit of this approach is that almost no changes are needed and we get compatible types between dcalc and lcalc, allowing to deduplicate a few functions.
It might not be the best in the long run: there are still benefits in factorising small parts of the AST as suggested in #157, and this forces a central AST definition that makes the nanopass-like approach a bit less legible.
Still, I think it's a step in the right direction and it doesn't really lock us in keeping to use the big GADT (as the minimal cascade of changes show).
