Implemented flash string

flash.hpp

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <avr/pgmspace.h>
+
+#define F(str) (reinterpret_cast<const detail::FlashString *>(PSTR(str)))
+
+namespace detail {
+
+// Only used for C++ type safety, because otherwise a PSTR would be indistinguishable from a normal c-string
+struct FlashString;
+
+} // namespace detail