Ensure relevant processes are garbage collected before reevaluation

[Jonatan Kłosko]

Given a cell like this:

Kino.start_child({DynamicSupervisor, name: :my_supervisor})

Currently it returns {:error, {:already_started, pid}} on some reevaluations, because the process from previous reevaluation is not killed on time. We need to garbage collect processes synchronously. We need to somehow integrate this into the referencing&monitoring mechanism we have.