We used only 6 bit to save the short size but for SmallString with a size
over 64 it is not enough. So we have now to use a uint16 instead of a
uint8 if the size if over 64.

Change-Id: I53558e492b6cb40b739b23a8af83d192a2e11bd2
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>