Support return position impl trait in trait

[Nadrieril]

E.g.

trait Foo<T> {
    fn iter<'a>(&'a self) -> impl Iterator<Item=&'a T>;
}

[Nadrieril]

I think for the most part this requires tracking items not by name (RPITIT items don't have a name) but by some new id. I'm thinking AssocTyId/AssocConstId/MethodId for the various kinds of trait items.