Emu: Convert Emu class to singleton

src/emu.h

@@ -2,10 +2,14 @@
 
 #include <cstdint>
 #include <vector>
-#include "processing_unit.h"
 
-class Emu {
+#include "processing-unit.h"
+#include "ruc/singleton.h"
+
+class Emu : ruc::Singleton<Emu> {
 public:
+	Emu(s) {}
+
 	void WriteRAM(int location, int length);
 	void WriteVRAM(int location, int length);
 	void WriteROM(int location, int length);