Override [ASCellNode supernode] to return its ASTableView's node wrapper

This may or may not be the best solution, but it is the simplest.

This fix allows `_ASDisplayNodeFindClosestCommonAncestor` to traverse `ASTableView` hierarchies without throwing an exception.

An alternative fix would be to add a check inside `_ASDisplayNodeFindClosestCommonAncestor` like `if ([supernode isKindOfClass:[ASCellNode class]])` and *then* jump up to the `ASTableView` node wrapper instead of crashing.

Thoughts? :smile: 
This is pretty brittle.  ASCellNode shouldn't make assumptions about the view hierarchy — if it changes, this code will break without warning.  If we believe this shimming is necessary, we should find a less-hacky solution.