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, '_>
```