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/ai_sl.cpp

@@ -30,7 +30,6 @@ static const SaveLoad _ai_company[] = {
 	   SLEG_SSTR(_ai_saveload_settings,     SLE_STR),
 	SLEG_CONDVAR(_ai_saveload_version,   SLE_UINT32, SLV_108, SL_MAX_VERSION),
 	SLEG_CONDVAR(_ai_saveload_is_random,   SLE_BOOL, SLV_136, SL_MAX_VERSION),
-	     SLE_END()
 };
 
 static void SaveReal_AIPL(int *index_ptr)