Once the handlers were moved away into its own separate module,
it makes no sense to keep the types of some of the handlers' fields
in other modules (such as causation).

Instead, it should be all together with handlers — and so we can move
the whole thing to the `kopf.structs` package eventually (next commit).
Because all of them are structs after all, not the reactor's processes.

Because they are structs, after all. Also, in the few following
commits, the daemons structs will be introduced, and they need
to import and use handlers, from which they were spawned. It would
be problematic to import a high-level `reactor.handlers` from
a low-level `structs.containers` (if not moved).

Originally, the hope was to reuse them in daemons & timers. But this
breaks the semantics of the handlers: patches should be applied
on every exit, not only on successes; multiple restarts should
be counted & limited and distributed with backoffs; extra daemon
restarts are not needed at all if regular retrying works; etc.