Codechange: make [Save|Load]Settings() behave more like other Save/Load code (#9335)

Prepare the full description and send it to SlObject. This does
require some code to be able to read to a SLE_VAR_NULL, like strings
etc, as there is no way to know their length beforehand.

src/saveload/saveload.h

@@ -893,7 +893,6 @@ void WriteValue(void *ptr, VarType conv, int64 val);
 void SlSetArrayIndex(uint index);
 int SlIterateArray();
 
-uint SlCalcConvMemLen(VarType conv);
 void SlAutolength(AutolengthProc *proc, void *arg);
 size_t SlGetFieldLength();
 void SlSetLength(size_t length);