Remove the dependency on GMP

[BerryYoghurt]

I am currently in the process of transpiling this library to C [see here], but we believe that having GMP as a dependency might cause some complications. Until now, there hasn't been much difficulty; but because GMP is itself C ported to Rust, it might get more complicated later on to compile C which has been transpiled from Rust which itself uses C.

So could you please replace rug with some pure Rust big integer library? Thanks!

[elichai]

Done