This adds a new IStatefulUnaryOperator interface, which differs from IUnaryOperator simply in that its Invoke members are instance rather than static. It then copies the existing InvokeSpanIntoSpan into a new helper that's then tweaked to accept an instance of an IStatefulUnaryOperator, calling its instance members instead of its static.

That helper is then used to implement new ShiftLeftOperator, ShiftRightLogicalOperator, ShiftRightArithmeticOperator, RotateLeftOperator, and RotateRightOperator, which utilize methods on VectorXx to accelerate their implementation.

It also adds ScaleBOperator, RootNOperator, and RoundOperator implementations used with the same helper, but these currently only have scalar implementations.