decoder/dsdlib: add class DsdUint64

Merge lots of duplicate code.

src/decoder/DsdLib.hxx

@@ -20,6 +20,7 @@
 #ifndef MPD_DECODER_DSDLIB_HXX
 #define MPD_DECODER_DSDLIB_HXX
 
+#include "system/ByteOrder.hxx"
 #include "Compiler.h"
 
 #include <stddef.h>
@@ -35,6 +36,17 @@ struct DsdId {
 	bool Equals(const char *s) const;
 };
 
+class DsdUint64 {
+	uint32_t lo;
+	uint32_t hi;
+
+public:
+	constexpr uint64_t Read() const {
+		return (uint64_t(FromLE32(hi)) << 32) |
+			uint64_t(FromLE32(lo));
+	}
+};
+
 bool
 dsdlib_read(Decoder *decoder, InputStream &is,
 	    void *data, size_t length);