With these changes, I'd also like to propose that we move ASNodeController
out of Beta (renaming the files without +Beta). Let me know what you think!
Because we don't support ASNodeController directly in ASCV / ASTV, it is still
important to allow flipping the ownership in certain cases (in particular, for
root CellNodeController objects that should follow the lifecycle of the
ASCellNode rather than owning the ASCellNode).