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/map_sl.cpp

@@ -20,10 +20,9 @@
 static uint32 _map_dim_x;
 static uint32 _map_dim_y;
 
-static const SaveLoadGlobVarList _map_dimensions[] = {
+static const SaveLoad _map_dimensions[] = {
 	SLEG_CONDVAR(_map_dim_x, SLE_UINT32, SLV_6, SL_MAX_VERSION),
 	SLEG_CONDVAR(_map_dim_y, SLE_UINT32, SLV_6, SL_MAX_VERSION),
-	    SLEG_END()
 };
 
 static void Save_MAPS()