Inlining Exprs in `Sentence`s without Expr data constructors

[balacij]

Resulting as a subtask from #2893, which has a bit of discussion of the context

I think the trick would be to have Sentence have a parameter or two. Or at least SentenceF would, and then Sentence for external consumption would "close over" particular parameters of SentenceF.

Originally posted by @JacquesCarette in https://github.com/JacquesCarette/Drasil/issues/2893#issuecomment-968295439

[JacquesCarette]

The point would be that there would be a 'hole' in SentenceF that could be filled by anything. That way SentenceF would be parametric in that, and would not depend on Expr. When we create Sentence, which could be in an entirely different package, it can fill the hole of SentenceF with Expr.

That should let us decouple various things (and reveal bad assumptions too).