BigInt type for Leema

[mdg]

Leema needs a bigint type for doing fun math stuff with big numbers
Rug seems like the most likely, partly b/c it's build on old gnu stuff. The other alternative might be rust-num/big-in which appears to be entirely in Rust.

[itscomputers]

@mdg what do you think about this thing i was doing for my rust number theory package? could leema use something like this?
https://github.com/itscomputers/ebe-rust/blob/master/src/types/int.rs
it uses a rust big-int but makes it compatible (arithmetic-wise) with all integer types.
(also keep in mind that i'm a rust newbie, so some of it might be shit lol)