Further struct work, fix lots of warnings

src/types.h

@@ -234,10 +234,10 @@ void appendToList(List* list, GroundValue value);
 
 // Gets item at index (idx) from list (list). If there is an error, it
 // will be indicated in the status field.
-ListAccess getListAt(List* list, int idx);
+ListAccess getListAt(List* list, size_t idx);
 
 // Sets an item in list (list) at index (idx) to GroundValue (value).
-ListAccessStatus setListAt(List* list, int idx, GroundValue value);
+ListAccessStatus setListAt(List* list, size_t idx, GroundValue value);
 
 // Creates a Ground struct
 GroundStruct createStruct();