Stabilize Future adapters and IntoFuture

[yoshuawuyts]

Stabilizes several stream adapters that have been proven over the last year. Also stabilizes IntoFuture. Thanks!

Changelog

## Changed

- `future::Future::delay` has been stabilized
- `future::Future::flatten` has been stabilized
- `future::Future::race` has been stabilized
- `future::Future::try_race` has been stabilized
- `future::Future::join` has been stabilized
- `future::Future::try_join` has been stabilized
- `future::Future::timeout` has been stabilized
- `future::Future::delay` has been stabilized
- `future::IntoFuture` has been stabilized

[yoshuawuyts]

CI is failing because of #883

[Keruspe]

Btw, should we switch to libstd's IntoFuture or would that be breaking?

[dignifiedquire]

@yoshuawuyts these CI failures look somewhat real to me