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/cargomonitor_sl.cpp

@@ -24,7 +24,6 @@ struct TempStorage {
 static const SaveLoad _cargomonitor_pair_desc[] = {
 	SLE_VAR(TempStorage, number, SLE_UINT32),
 	SLE_VAR(TempStorage, amount, SLE_UINT32),
-	SLE_END()
 };
 
 static CargoMonitorID FixupCargoMonitor(CargoMonitorID number)