## Summary
As discussed on ~IRC~ Discord, this will make it easier for e.g. the
docs generation stuff to get all rules for a linter (using
`all_rules()`) instead of just non-nursery ones, and it also makes it
more Explicit Is Better Than Implicit to iterate over linter rules.
Grepping for `Item = Rule` reveals some remaining implicit
`IntoIterator`s that I didn't feel were necessarily in scope for this
(and honestly, iterating over a `RuleSet` makes sense).