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/storage_sl.cpp

@@ -18,7 +18,6 @@ static const SaveLoad _storage_desc[] = {
 	 SLE_CONDVAR(PersistentStorage, grfid,    SLE_UINT32,                  SLV_6, SL_MAX_VERSION),
 	 SLE_CONDARR(PersistentStorage, storage,  SLE_UINT32,  16,           SLV_161, SLV_EXTEND_PERSISTENT_STORAGE),
 	 SLE_CONDARR(PersistentStorage, storage,  SLE_UINT32, 256,           SLV_EXTEND_PERSISTENT_STORAGE, SL_MAX_VERSION),
-	 SLE_END()
 };
 
 /** Load persistent storage data. */