Breaking changes

- The `Dispatcher` type no longer has the closure type within its type parameters,
  but instead now has an explicit lifetime parameter, as well as the source type `S`
  and the event loop `Data` type. This allows the type to be explicitly named and
  stored into an other struct.

Additions

- `Token` now has a method `with_sub_id()` that returns a copy of the token
  but with the given `sub_id`.