Separate trait items from trait

[nikomatsakis]

PR #37660 separates impl items from impls in the HIR for improved interaction with incremental compilation. It probably makes sense to separate trait items from traits too -- not so much for improved incremental compilation precision, as because it makes the layout more analogous.

[nikomatsakis]

Marking this as E-mentor. I'll give a short write-up here; happy to provide a more detailed one too upon request!

The work that needs to be done is basically to model trait items on impl items -- we would get a hir::TraitItemRef like hir::ImplItemRef, a separate map in the Krate to store them, and modify the lowering routines analogously. Finally, there are a few places (marked with FIXMEs attached to this issue number) where we could convert to using a ItemLikeVisitor if trait items were considered "item like".

[eddyb]

Apologies to anyone who wanted to pick this up, but I was blocked on it so I did it, PR coming soon.