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/cargopacket_sl.cpp

@@ -83,7 +83,7 @@
  * some of the variables itself are private.
  * @return the saveload description for CargoPackets.
  */
-const SaveLoad *GetCargoPacketDesc()
+SaveLoadTable GetCargoPacketDesc()
 {
 	static const SaveLoad _cargopacket_desc[] = {
 		     SLE_VAR(CargoPacket, source,          SLE_UINT16),
@@ -97,8 +97,6 @@ const SaveLoad *GetCargoPacketDesc()
 
 		/* Used to be paid_for, but that got changed. */
 		SLE_CONDNULL(1, SL_MIN_VERSION, SLV_121),
-
-		SLE_END()
 	};
 	return _cargopacket_desc;
 }