Change: prefix SL_ARR with the length of the array

This means that during loading we can validate that what is saved
is also that what is expected. Additionally, this makes all list
types similar to how they are stored on disk:
First a gamma to indicate length, followed by the data.
The size still depends on the type.

src/saveload/saveload.h

@@ -1008,7 +1008,7 @@ byte SlReadByte();
 void SlWriteByte(byte b);
 
 void SlGlobList(const SaveLoadTable &slt);
-void SlArray(void *array, size_t length, VarType conv);
+void SlCopy(void *object, size_t length, VarType conv);
 void SlObject(void *object, const SaveLoadTable &slt);
 void NORETURN SlError(StringID string, const char *extra_msg = nullptr);
 void NORETURN SlErrorCorrupt(const char *msg);