Codechange: C++-ify lists for SaveLoad (#9323)

Basically, this changes "SaveLoad *" to either:
1) "SaveLoadTable" if a list of SaveLoads was meant
2) "SaveLoad &" if a single entry was meant

As added bonus, this removes SL_END / SLE_END / SLEG_END. This
also adds core/span.hpp, a "std::span"-lite.

src/order_backup.h

@@ -15,6 +15,7 @@
 #include "tile_type.h"
 #include "vehicle_type.h"
 #include "base_consist.h"
+#include "saveload/saveload.h"
 
 /** Unique identifier for an order backup. */
 typedef uint8 OrderBackupID;
@@ -34,7 +35,7 @@ static const uint32 MAKE_ORDER_BACKUP_FLAG = 1U << 31;
  */
 struct OrderBackup : OrderBackupPool::PoolItem<&_order_backup_pool>, BaseConsist {
 private:
-	friend const struct SaveLoad *GetOrderBackupDescription(); ///< Saving and loading of order backups.
+	friend SaveLoadTable GetOrderBackupDescription(); ///< Saving and loading of order backups.
 	friend void Load_BKOR();   ///< Creating empty orders upon savegame loading.
 	uint32 user;               ///< The user that requested the backup.
 	TileIndex tile;            ///< Tile of the depot where the order was changed.