The goal here is to make this code more readable and less ad-hoc.

For a GAT like

```
impl<T> Foo<T> { type Bar<'a> = ..}
```

Instead of constructing nested quantifiers like

```
forall<T> {
  if (..) {
    stuff &&
    forall<'a> { more_stuff }
  }
}
```

we will now build separate goals

```
forall<T> { if (...) { stuff } } &&
forall<T, 'a> { if (...) { more_stuff } }
```

This both removes the need for the 'partial' forall methods in
GoalBuilder butalso means we would be able to give better error
messages in rustc at somepoint, since we can test "is this impl header
WF" separately from "is this associated type value WF" etc.