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/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: