## Motivation
Like for `Subscriber` and `Layer`, allow per-layer `Filter`s to filter
based on event fields.
## Solution
Add `Filter::event_enabled`, plumb it through the combinator
implementations, and call it from `Filtered`.
The bit I'm the least confident about is the check in `Registry`'s
implementation, but I *think* it matches what `event` is doing and
everything seems to function correctly.