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/script/script_instance.cpp

@@ -343,7 +343,6 @@ static byte _script_sl_byte; ///< Used as source/target by the script saveload c
 /** SaveLoad array that saves/loads exactly one byte. */
 static const SaveLoad _script_byte[] = {
 	SLEG_VAR(_script_sl_byte, SLE_UINT8),
-	SLE_END()
 };
 
 /* static */ bool ScriptInstance::SaveObject(HSQUIRRELVM vm, SQInteger index, int max_depth, bool test)