Use prefixes, import functionality.

hungry-delete-forward and -backward are extensions of delete-forward-char and delete-backward-char. Recent patches took into account that those original versions work in Delete Selection mode / Transient Mark mode. This patch takes into account that the originals use prefix arguments.

Not only does this allow deleting a specific number of characters (whitespace or otherwise), but it also means that Hungry can basically mimic all of the stock features of the original versions, including untabifying with Overwrite mode.