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/waypoint_sl.cpp

@@ -180,8 +180,6 @@ static const SaveLoad _old_waypoint_desc[] = {
 	SLE_CONDVAR(OldWaypoint, localidx,   SLE_UINT8,                   SLV_3, SL_MAX_VERSION),
 	SLE_CONDVAR(OldWaypoint, grfid,      SLE_UINT32,                 SLV_17, SL_MAX_VERSION),
 	SLE_CONDVAR(OldWaypoint, owner,      SLE_UINT8,                 SLV_101, SL_MAX_VERSION),
-
-	SLE_END()
 };
 
 static void Load_WAYP()