Move C/C++ sources to src subdirectory

src/base64.h

@@ -0,0 +1,8 @@
+// Copyright (c) Vitaliy Filippov, 2019+
+// License: VNPL-1.1 (see README.md for details)
+
+#pragma once
+#include <string>
+
+std::string base64_encode(const std::string &in);
+std::string base64_decode(const std::string &in);