untested saving code and memory leak fix

src/engine/song.cpp

@@ -0,0 +1,18 @@
+#include "song.h"
+
+void DivSong::unload() {
+  for (DivInstrument* i: ins) {
+    delete i;
+  }
+  ins.clear();
+
+  for (DivWavetable* i: wave) {
+    delete i;
+  }
+  wave.clear();
+
+  for (DivSample* i: sample) {
+    delete i;
+  }
+  sample.clear();  
+}