Add projection obligations when comparing impl too
Fixes#115033
In the test, when we ask for WF obligations of `DatasetIter<'a, ArrayBase<D>>`, we get back two important obligations: `[<D as Data>::Elem -> ?1, ?1: 'a]`. If we don't add the projection obligation, `?1` remains unconstrained.
An alternative solution would be to use unnormalized obligations, where we only have one relevant obligation: `<D as Data>::Elem: 'a`. This would leave no inference vars unconstrained.
fe5f591257