two ways to get RPITs into non-self positions in their item-bounds

```rust
trait DefaultParam<R: ?Sized = Self> {}
impl<T> DefaultParam for T {}
fn foo() -> impl DefaultParam {}

trait Super<T: ?Sized> {}
impl<T, U> Super<T> for U {}
trait Sub: Super<Self> {}
impl<T> Sub for T {}
fn bar() -> impl Sub {}
```

currently unstable ways:
- TAIT
- trait aliases