add Expr.split_{conjunctions,disjunctions}

[redianthus]

This will be useful in Owi to make the PC independence algorithm better, for now, a conjunction is forced to be put under the same union-find group, which is unfortunate. Splitting them this way before adding them to the UF should improve things a lot.

I also added split_disjunctions while I was at it, but not sure it'll be useful for now.

It may be possible to push the split even further, for instance, if we have e defined as $¬(P ∨ Q)$ we could do split_conjunctions e to be $\lbrace ¬P ; ¬Q \rbrace $. But I am not sure yet if this belong to this function or to a simplification, so I left it out for now. I'll experiment with this and see what would be best.

[redianthus]

@filipeom, should be OK now :)

[filipeom]

Thanks! 🙏