Added option to skip checks on UniqueSortIndex

[kcajf]

This is because I'm usually dealing with a known-sorted array, and want an easy interface to constructing an AcceleratedArray without any of the sorting & checks.

In terms of the interface: having a bool argument seemed quick and easy. Could instead add a new type PreSorted or something and dispatch on that as an argument, which would give type inference without relying on constant propagation.

I also thought about doing this for SortIndex, but noticed that it has the n_unique field + computation which makes things tricky. That field doesn't actually seem to ever be used in the package - could we ditch it?

[andyferris]

I'm torn between keyword and positional argument here. I can't imagaine what else we could do with a 3-arg form and it's possible to deprecate this, so this'll do :)