Use dynamic dispatch to simplify reporters (#10086)
## Summary
Sort of undecided on this. These are already stored as `dyn Reporter` in
each struct, so we're already using dynamic dispatch in that sense. But
all the methods take `impl Reporter`. This is sometimes nice (the
callsites are simpler?), but it also means that in practice, you often
_can't_ pass `None` to these methods that accept `Option<impl
Reporter>`, because Rust can't infer the generic type.
Anyway, this adds more consistency and simplifies the setup by using
`Arc<dyn Reporter>` everywhere.
