Previously, only Self, &Self, &mut Self, Arc<Self>, Rc<Self>,
and Box<Self> were available as stable method receivers.
This commit stabilizes nested uses of all the above types.
However, nested receivers remain non-object-safe.
We now always make fresh lifetimne parameters for all elided
lifetimes, whether they are in the inputs or outputs. But then
we generate `'_` in the case of elided lifetimes from the outputs.
Example:
```rust
async fn foo<'a>(x: &'a u32) -> &u32 { .. }
```
becomes
```rust
type Foo<'a, 'b> = impl Future<Output = &'b u32>;
fn foo<'a>(x: &'a u32) -> Foo<'a, '_>
```
