refactor!: rename SynthService to Synther and related types

The -er suffix is more idiomatic for single method interfaces, and
the interface is not doing much more than converting the patch to a
synth. Names were updated throughout the project to reflect this
change. In particular, the "Service" in SynthService was not telling
anything helpful.

cmd/sointu-play/main.go

@@ -88,7 +88,7 @@ func main() {
 				return fmt.Errorf("the song could not be parsed as .json (%v) or .yml (%v)", errJSON, errYaml)
 			}
 		}
-		buffer, err := sointu.Play(bridge.BridgeService{}, song, !*unreleased) // render the song to calculate its length
+		buffer, err := sointu.Play(bridge.NativeSynther{}, song, !*unreleased) // render the song to calculate its length
 		if err != nil {
 			return fmt.Errorf("sointu.Play failed: %v", err)
 		}