Replace AsyncExecutor with simple closure

This makes everything much simpler. The task type is now defined
directly on `Plugin`.

plugins/examples/midi_inverter/src/lib.rs

@@ -33,8 +33,7 @@ impl Plugin for MidiInverter {
     const MIDI_OUTPUT: MidiConfig = MidiConfig::MidiCCs;
     const SAMPLE_ACCURATE_AUTOMATION: bool = true;
 
-    type AsyncExecutor = ();
-    fn async_executor(&self) -> Self::AsyncExecutor {}
+    type BackgroundTask = ();
 
     fn params(&self) -> Arc<dyn Params> {
         self.params.clone()