Can TaskLocalFuture be public?

[NeoLegends]

Is your feature request related to a problem? Please describe.
We'd like to use task locals from nameable futures (to use them with Service w/o boxing), however, LocalKey::scope uses async fn which stands between us and the goal of being able to name futures.

Describe the solution you'd like
LocalKey::scope already constructs a hand-implemented future. It's just not exposing that to the outside but awaiting it directly. Ideally the function would just return the TaskLocalFuture instead of awaiting it directly.

I think the fact that the future is private is because of the StaticLifetime-trait that tokio does not want to expose, because it's used as a workaround for pin_project_lite limitations. If we sealed that trait, however, exposing it would be fine.

If you agree this is a solution I'm happy to file a PR for both tokio 0.3 and 0.2.

Describe alternatives you've considered
We can just box all our futures that add task locals to their inner futures, or wait until impl Trait in associated type position / async fn in traits becomes stable.

Kind of moot to always keep waiting for Rust features until actually doing something, though. 😄

Additional context
Implementing tower::Service.