Fix Set indices on datatypes not being caught

[zeroeightysix]

compileDomType would return DomDropped for types universally quantified in Haskell, but the erased indices check would understand that as the type being erased; which is not the case for Set indices, as I understand it. Is this the right solution?

I've also added a test for this case.

Fixes #437

[jespercockx]

Hmm, it makes sense to make a more explicit distinction between an erased domain and one that is just left implicit in the Haskell code. However, I think it would make more sense to treat the latter as a DomForall rather than a DomDropped, so it would be better to add an argument to the DomForall constructor (do we want to see this forall explicitly in the generated Haskell code or not?) rather than to the DomDropped constructor.

[zeroeightysix]

That does make more sense! I've redone the changes, could you look at them again?

[jespercockx]

It's perfect now, thank you for the PR!