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/labelmaps_sl.cpp

@@ -95,7 +95,6 @@ struct LabelObject {
 
 static const SaveLoad _label_object_desc[] = {
 	SLE_VAR(LabelObject, label, SLE_UINT32),
-	SLE_END(),
 };
 
 static void Save_RAIL()