feat: business invariants for NonEmptySet

[sterraf]

WIP on defining a BusinessInvariants trait. For the particular case of NonEmptySet, it checks that the inner Vec is nonempty and contains no repetitions. For the last check, I'm using BTreeSet as a proxy, but this can be obviously changed to HashSet.

The trait provides the Boolean validate_data method; another option would be to return a Result<(), E> where E describes more precisely the problems encountered.