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/loudness_war_winner/src/lib.rs

@@ -123,6 +123,7 @@ impl Plugin for LoudnessWarWinner {
     const DEFAULT_INPUT_CHANNELS: u32 = 2;
     const DEFAULT_OUTPUT_CHANNELS: u32 = 2;
 
+    type SysExMessage = ();
     type BackgroundTask = ();
 
     fn params(&self) -> Arc<dyn Params> {