refactor links to be a top-level part of any Spear.Event

[the-mikedavis]

can be nil when there is no link

%Spear.Event{
  id: uuid,
  type: "event_type",
  body: %{"foo" => "bar"},
  metadata: %{..},
  link: nil
}

Ideally the shape of this :link field would also be a Spear.Event