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/crossover/src/lib.rs

@@ -183,6 +183,7 @@ impl Plugin for Crossover {
         aux_outputs: Some(&["Band 1", "Band 2", "Band 3", "Band 4", "Band 5"]),
     };
 
+    type SysExMessage = ();
     type BackgroundTask = ();
 
     fn params(&self) -> Arc<dyn Params> {