Add "rug" to Big Integer

[mabbamOG]

rug (https://docs.rs/rug/1.11.0/rug/) is a crate that ports GMP into idiomatic Rust types and traits. It performs very well, and makes use of decades of work provided by the underlying and well-established GNU GMP library. It should definitely be considered as a contender for "num", at the very least for performance concerns, although it may not be as idiomatic as num. Performance is at least 2x if not much more