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/main_synther_native.go

@@ -4,4 +4,4 @@ package cmd
 
 import "github.com/vsariola/sointu/vm/compiler/bridge"
 
-var DefaultService = bridge.BridgeService{}
+var MainSynther = bridge.NativeSynther{}

cmd/main_synther_not_native.go

@@ -4,4 +4,4 @@ package cmd
 
 import "github.com/vsariola/sointu/vm"
 
-var DefaultService = vm.SynthService{}
+var MainSynther = vm.GoSynther{}

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)
 		}

cmd/sointu-track/main.go

@@ -57,8 +57,8 @@ func main() {
 		recoveryFile = filepath.Join(configDir, "Sointu", "sointu-track-recovery")
 	}
 	model := tracker.NewModel(modelMessages, playerMessages, recoveryFile)
-	player := tracker.NewPlayer(cmd.DefaultService, playerMessages, modelMessages)
-	tracker := gioui.NewTracker(model, cmd.DefaultService)
+	player := tracker.NewPlayer(cmd.MainSynther, playerMessages, modelMessages)
+	tracker := gioui.NewTracker(model, cmd.MainSynther)
 	output := audioContext.Output()
 	defer output.Close()
 	go func() {

cmd/sointu-vsti/main.go

@@ -62,8 +62,8 @@ func init() {
 			recoveryFile = filepath.Join(configDir, "Sointu", "sointu-vsti-recovery-"+hex.EncodeToString(randBytes))
 		}
 		model := tracker.NewModel(modelMessages, playerMessages, recoveryFile)
-		player := tracker.NewPlayer(cmd.DefaultService, playerMessages, modelMessages)
-		tracker := gioui.NewTracker(model, cmd.DefaultService)
+		player := tracker.NewPlayer(cmd.MainSynther, playerMessages, modelMessages)
+		tracker := gioui.NewTracker(model, cmd.MainSynther)
 		tracker.SetInstrEnlarged(true) // start the vsti with the instrument editor enlarged
 		go tracker.Main()
 		context := VSTIProcessContext{make([]vst2.MIDIEvent, 100), h}