Move flow and function opcodes into their own file

src/opcodes/flow.cpp

@@ -0,0 +1,20 @@
+/* This file is part of the sirit project.
+ * Copyright (c) 2018 ReinUsesLisp
+ * This software may be used and distributed according to the terms of the GNU
+ * General Public License version 2 or any later version.
+ */
+
+#include "sirit/sirit.h"
+#include "opcodes.h"
+
+namespace Sirit {
+
+const Op* Module::Label() {
+    return AddCode(spv::Op::OpLabel, bound++);
+}
+
+const Op* Module::Return() {
+    return AddCode(spv::Op::OpReturn);
+}
+
+} // namespace Sirit