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/linkgraph/linkgraphschedule.h

@@ -39,7 +39,7 @@ private:
 	~LinkGraphSchedule();
 	typedef std::list<LinkGraph *> GraphList;
 	typedef std::list<LinkGraphJob *> JobList;
-	friend const SaveLoad *GetLinkGraphScheduleDesc();
+	friend SaveLoadTable GetLinkGraphScheduleDesc();
 
 protected:
 	ComponentHandler *handlers[6]; ///< Handlers to be run for each job.