Allow multiple #[unstable] attributes on one item

[m-ou-se]

We sometimes have things that should be gated by multiple features. E.g. ScopedJoinHandle::is_running is part of scoped_threads, but also of thread_is_running. In these cases we now just apply only one of the #[unstable] attributes, and because of that sometimes accidentally stabilize something when only one of the features gets stabilized.

It'd be great if we could add multiple #[unstable] attributes to a single item. Allowing multiple #[stable] tags isn't a necessity. We could just remove the #[unstable] tag if the feature gets stabilized and there are other #[unstable] tags left. The most important part is that we don't forget about the other unstable tags and don't stabilize something by accident.

Another option would be multiple feature = arguments to #[unstable].

[compiler-errors]

@rustbot claim