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/saveload/saveload_internal.h

@@ -23,7 +23,7 @@ void ResetOldNames();
 void ResetOldWaypoints();
 void MoveBuoysToWaypoints();
 void MoveWaypointsToBaseStations();
-const SaveLoad *GetBaseStationDescription();
+SaveLoadTable GetBaseStationDescription();
 
 void AfterLoadVehicles(bool part_of_load);
 void FixupTrainLengths();