td: Handle name changes.

[betterengineering]

This commit updates the change name to try to use the To name first. Given we skip deletes, the to field should always be populated. The library code in question:

type Change struct {
	From ChangeEntry
	To   ChangeEntry
}

var empty ChangeEntry

// Action returns the kind of action represented by the change, an
// insertion, a deletion or a modification.
func (c *Change) Action() (merkletrie.Action, error) {
	if c.From == empty && c.To == empty {
		return merkletrie.Action(0),
			fmt.Errorf("malformed change: empty from and to")
	}

	if c.From == empty {
		return merkletrie.Insert, nil
	}

	if c.To == empty {
		return merkletrie.Delete, nil
	}

	return merkletrie.Modify, nil
}