Better support for unsized values in Pearlite

[Lysxia]

JH's idea: wrap the result of all #[logic] functions in Box so users can write logic functions that return unsized types.

[jhjourdan]

I tried this, and I am going to abandon, and close this issue.

The reason for this is that I encountered several problems which I do not know how to solve. Namely:

• Pearlite code may call functions for which we cannot wrap the return type in Box/&. This include ADT constructors, Box::new and Snapshot::deref/deref_mut, and we don't have a reliable way to distinguish these withthe information we have at the macro level.
• If we merge Logical functions aliases #1346 one day, the aliased program functions will have the same problem
• If the goal were to only support ADT conostructors and wrapped logic functions, then we could use a generic trait method &Self -> &Self and use the auto-ref mechanism after each function call, but then we would not benefit from top-down type propagation, making it mandatory to annotate the type of functions like such_that.

If one day we find a workaround of all these issues, then we may reopen this, but for now I consider this as a bad idea.