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/linkgraph.h

@@ -16,10 +16,10 @@
 #include "../station_base.h"
 #include "../cargotype.h"
 #include "../date_func.h"
+#include "../saveload/saveload.h"
 #include "linkgraph_type.h"
 #include <utility>
 
-struct SaveLoad;
 class LinkGraph;
 
 /**
@@ -525,8 +525,8 @@ public:
 protected:
 	friend class LinkGraph::ConstNode;
 	friend class LinkGraph::Node;
-	friend const SaveLoad *GetLinkGraphDesc();
-	friend const SaveLoad *GetLinkGraphJobDesc();
+	friend SaveLoadTable GetLinkGraphDesc();
+	friend SaveLoadTable GetLinkGraphJobDesc();
 	friend void SaveLoad_LinkGraph(LinkGraph &lg);
 
 	CargoID cargo;         ///< Cargo of this component's link graph.

src/linkgraph/linkgraphjob.h

@@ -52,7 +52,7 @@ private:
 	typedef std::vector<NodeAnnotation> NodeAnnotationVector;
 	typedef SmallMatrix<EdgeAnnotation> EdgeAnnotationMatrix;
 
-	friend const SaveLoad *GetLinkGraphJobDesc();
+	friend SaveLoadTable GetLinkGraphJobDesc();
 	friend class LinkGraphSchedule;
 
 protected:

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.