What this patch does:
- Stability annotations are now based on "exported items" supplied by rustc_privacy and not "public items". Exported items are as accessible for external crates as directly public items and should be annotated with stability attributes.
- Trait impls require annotations now.
- Reexports require annotations now.
- Crates themselves didn't require annotations, now they do.
- Exported macros are annotated now, but these annotations are not used yet.
- Some useless annotations are detected and result in errors
- Finally, some small bugs are fixed - deprecation propagates from stable deprecated parents, items in blocks are traversed correctly (fixes https://github.com/rust-lang/rust/issues/29034) + some code cleanup.
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