Removed bigint dependency and changed RSA algorithm to use u128

[mgostIH]

This removes a big dependency by using Rust's available u128 for the RSA step. I checked that tests pass and the change is quite simple. Should reduce compile times quite a lot!

[mlafeldt]

Hey @mgostIH, thanks for the PR!

I actually don't mind longer compile times if the code is more readable and maintainable by using dedicated crates. That said, I'm not sure if I should merge this change or not.

In any case, it's good to see that it can be done with u128. 👍

Mind telling me where exactly you copied that mod_pow from?

[mgostIH]

From Rust's pow and just added a couple of % operations: https://doc.rust-lang.org/src/core/num/mod.rs.html#3908-3928
Also another reason for using this code is that it should allow for no_std, but I am too sure on what kind of operations you do elsewhere that would prevent it

[mlafeldt]

Ah! The crate already supports no_std, see #1

[mgostIH]

Indeed, but it would still require the alloc crate, while from what I am seeing this whole crate could work without it, it's only used inside tests for vec! but it could be easily be removed. The advantage of removing alloc is that someone using your crate externally wouldn't require to set up a fixed allocator or something basic, and expecially if it's for small devices that work directly on the PS2 it would be an additional benefit.
An additional way to make this code simpler would instead of mutating input arguments to return a tuple (u32, u32), possibly a Result too in order to detect the case where there was a failure in decoding the values due to the check of code < modulus

[mlafeldt]

I justed merged #4, which removes the last remaining vector. So now, String and Vec are only needed for tests. :)

[mgostIH]

So are you going to merge this and remove the usage of the heap entirely?

[mlafeldt]

@mgostIH I noticed that the PR was automatically closed when I renamed master to main. That wasn't my intention, sorry.

That being said, I'm still going to stick with modpow function though.