FDS: Use retro_get_memory_data/size for disk saving

src/fds.h

@@ -1,6 +1,9 @@
 #ifndef _FCEU_FDS_H
 #define _FCEU_FDS_H
 
+uint8 *FDSROM_ptr(void);
+uint32 FDSROM_size(void);
+
 void FDSSoundReset(void);
 
 void FCEU_FDSInsert(int oride);