Random elements of large finite sets

For very big n generating random integers modulo n via Integer::structure().generate_random_elements().next().unwrap() is slow, because the current implementation creates a list of all elements and picks numbers out at random. It's done that way so it works with any FiniteSetSignature, but it is going to be slow for large sets where a small number of random elements are wanted. There should be more efficient implementations for those cases where possible.

Start at the Integer::structure().generate_random_elements() entry point and inspect how it currently creates a list of all elements before selecting values. Compare possible approaches for large finite sets while preserving support for any FiniteSetSignature; done means large sets can produce a small number of random elements more efficiently.