Add: BinaryInteger/doubled()

[oscbyspro]

BinaryInteger/times(2) is common but not ideal for arbitrary integers since it always creates a new allocation. It is equivalent to one addition-by-self, so consuming addition semantics attempting an in-place transformation might be more reasonable. Systems integers can derive it. In the case of arbitrary integers, it depends on whether /up(Shift.one) or addition-by-self is better. Just a thought at this point.

[oscbyspro]

The derived approach is a bit faster than times(2) for ≤ 100,000 bits. I would have thought it would always be faster. I suppose I'll have to take a closer look at up(Shift.one) at some point. I have not tried a custom self + self yet. But, perhaps a custom times(Element) would be better than a custom doubled()? Or, perhaps I can do something fancier with a future non-escapable /elements view? In any case, this is fine for now.