Add a SysExMessage type to Plugin

This is needed to enable sending and receiving SysEx #54. Because
associated type defaults still are not stable, this requires every
plugin that doesn't need this to set this to the unit type:

```rust
type SysExMessage = ();
```

plugins/examples/poly_mod_synth/src/lib.rs

@@ -155,6 +155,7 @@ impl Plugin for PolyModSynth {
     const MIDI_INPUT: MidiConfig = MidiConfig::Basic;
     const SAMPLE_ACCURATE_AUTOMATION: bool = true;
 
+    type SysExMessage = ();
     type BackgroundTask = ();
 
     fn params(&self) -> Arc<dyn Params> {