Breaking changes

- MSRV is now 1.41
- The `futures` module now has a proper error type for `Scheduler::schedule()`
- The return type of `EventSource::process_events()` is now `io::Result<PostAction>` allowing
  the sources to directly request the event loop to reregister/disable/destroy them.
- The `Token` creation mechanism is now driven by a `TokenFactory`, that
  dynamically generates new unique token for sub-sources. Following for this
  if you create a new event source that is not built by composing the ones
  provided by calloop, you need to check if the `Token` provided to
  `process_events` is the same as the one you created when (re)registering
  your source. If you delegate `process_events` to a sub-source, you no longer
  need to check the `sub_id` before, instead the source you are delegating to
  is responsible to to this check.

Bugfixes

- Cancelling a timeout no longer prevents later timeouts from firing.